65% of conversions start on one device and complete on another. Your customer clicks a Facebook ad on their phone during lunch. That night, they purchase from their laptop. Facebook counts the conversion. GA4 shows direct traffic. Neither is lying—they’re seeing different slices of the same journey through fundamentally different lenses.

This architectural difference explains why your GA4 conversion count never matches your ad platforms—and why chasing UTM fixes or consent mode settings won’t close the gap. The problem isn’t broken tracking. It’s how GA4 identifies users versus how Facebook and Google Ads identify users.

The Architecture of the Discrepancy

GA4 uses first-party cookies to identify users. These cookies are tied to specific browsers on specific devices. When a visitor lands on your site from mobile Safari, GA4 creates a cookie for that browser. When the same person visits from Chrome on their desktop, GA4 creates a different cookie. To GA4, these are two separate users.

Facebook tracks people using User ID rather than cookies. When someone is logged into Facebook on their phone and their laptop, Facebook knows it’s the same person across both devices. Mobile ad click to desktop purchase? Facebook connects them instantly.

According to Meta’s data shared via Ruler Analytics: “More than 65% of conversions start on one device and complete on another.” That’s not a rounding error. That’s the majority of your customer journeys happening in a way that GA4 fundamentally cannot track.

You may be interested in: Facebook Ads vs GA4: Why Revenue Numbers Never Match

What This Looks Like in Your Data

Your customer Sarah sees your Facebook ad while scrolling Instagram on her phone during her commute. She clicks, browses your products for three minutes, then closes the app—she’s at her stop. That night, she remembers your product, opens her laptop, types your URL directly, and completes the purchase.

Here’s how each platform sees this journey:

• Facebook Ads: Records the mobile click, matches Sarah’s User ID across devices, counts the conversion with full attribution to the ad campaign.
• GA4: Sees one mobile session (source: facebook/paid) that didn’t convert. Sees a separate desktop session (source: direct) that did convert. Credits the sale to “direct”—no connection to the ad spend.
• Google Ads: If you’re running Google campaigns, their logged-in user tracking would also catch cross-device journeys—but only for users signed into Google across devices.

GA4 treats mobile click and desktop purchase as two separate users unless signed into Google with Ads personalization. Most of your WooCommerce visitors aren’t logged into Google on both devices with personalization enabled. For them, GA4 is blind to the cross-device journey.

Why Mobile-Heavy Sectors Get Hit Hardest

Analysis from Eaglytics shows up to 80% discrepancy in mobile-heavy sectors where cross-device journeys are most common. If your customers discover products on mobile—through social ads, email clicks, or search—but convert on desktop, your GA4 systematically undercounts the channels that drive discovery.

The pattern is consistent:

• Facebook/Instagram ads: High mobile click rates + desktop purchase completion = GA4 misses the connection
• Email marketing: Mobile email opens + desktop purchase = GA4 often credits “direct”
• Organic social: Mobile discovery + desktop research/purchase = attribution lost

Every channel that performs well on mobile but converts on desktop gets systematically undervalued by GA4. Meanwhile, bottom-funnel channels that capture the final desktop click (brand search, direct, retargeting) get overvalued.

You may be interested in: Why Every Platform Claims Credit for Your Sales

Why This Isn’t Fixable (And That’s Okay)

Here’s the thing: this isn’t a bug. It’s not something you misconfigured. It’s the fundamental architecture of how these platforms track users.

GA4 was designed around first-party cookies because that’s what website analytics tools have access to. Google wanted GA4 to respect privacy boundaries and not require users to be logged in. The tradeoff is cross-device blindness for most visitors.

Facebook was designed around user accounts because that’s what social platforms have—persistent logged-in identities. The tradeoff is they can only track journeys where their user is involved.

Neither platform is “right” or “wrong.” They’re measuring different things:

• GA4 measures: Browser sessions and conversions attributable within a single device
• Facebook measures: User journeys across any device where they can identify the user

Expecting these numbers to match is expecting two different measurements of the same phenomenon to produce identical results. They won’t. Understanding why helps you stop chasing fixes that don’t exist.

How to Interpret the Gap Correctly

Once you understand cross-device blindness, you can interpret your data more accurately:

1. GA4 undercounts mobile-initiated channels. If Facebook says it drove 100 conversions and GA4 says 60, the gap isn’t “wrong”—it’s partially the 40% of journeys that crossed devices and GA4 couldn’t connect.

2. “Direct” traffic in GA4 includes cross-device conversions. When GA4 shows a spike in direct conversions, some portion represents users who discovered you elsewhere (on mobile) and completed the purchase on desktop without a trackable referrer.

3. Platform numbers will never match. Budget for 20-60% discrepancy depending on your mobile/desktop mix. If you’re within that range, your tracking probably isn’t broken—it’s working as designed, just measuring different things.

4. Bottom-funnel appears overvalued. Channels that capture the final click (brand search, direct, email resends) look artificially strong because they get credit for journeys that started elsewhere on a different device.

What You Can Do About It

You can’t fix the architectural difference between GA4 and Facebook. But you can improve cross-device matching by giving all platforms a consistent identifier to work with.

The identifier that crosses devices: customer email. When a user logs in or checks out, their email becomes a persistent identifier that every platform can recognize across devices. The key is sending this identifier (properly hashed) to all platforms simultaneously.

Server-side tracking with Transmute Engine™ captures purchase events at the WooCommerce hook level, then sends the same customer identifier (SHA256 hashed email) to GA4, Facebook CAPI, Google Ads, and BigQuery in a single server-side dispatch from your first-party subdomain. All platforms receive the same identifier at the same time.

This doesn’t magically make GA4 track cross-device journeys—that’s still limited by Google’s architecture. But it does:

• Improve Facebook’s cross-device matching by providing consistent identifiers server-side
• Enable BigQuery analysis where you can join customer data across sessions using email as a key
• Ensure all platforms receive the same data so discrepancies reflect architectural differences, not data collection gaps

Key Takeaways

• 65% of conversions cross devices—this is the primary driver of GA4 vs ad platform discrepancies, not broken tracking.
• GA4 uses browser cookies that cannot track the same user across devices. Facebook uses User ID that persists everywhere the user is logged in.
• Mobile-heavy sectors see up to 80% discrepancy because mobile discovery + desktop purchase breaks GA4 attribution.
• Stop trying to make the numbers match. They measure different things. Understand the gap instead of chasing phantom fixes.
• Server-side tracking with consistent identifiers improves cross-device matching on platforms that support it and enables BigQuery analysis across sessions.

Cross-device attribution isn’t broken—it’s architecturally impossible for cookie-based analytics to track. Understanding this helps you stop chasing fixes and start making better decisions with the data you have. See how Transmute Engine sends consistent identifiers to all platforms simultaneously.