Full Answer

The gap is attribution methodology, not fraud. Facebook's default 7-day click + 1-day view window means someone who sees your ad on Monday and buys via Google on Wednesday gets counted by Facebook. GA4 records a Google-attributed session and credits Google. Meta reports 26% higher conversions on average than analytics tools due to this modeled view-through attribution (Varos, 2024). GA4 also underreports paid conversions by 18–35% when cookies are rejected or blocked. Server-side tracking via Facebook CAPI narrows the gap by ensuring Facebook receives confirmed purchase events even when the browser pixel is blocked. The numbers will never fully match — each platform's attribution model reflects its own measurement philosophy and business interests.