Attribution Window Mismatch
Google Ads defaults to a 30-day click attribution window. GA4 uses a different session-based model. Meta runs on a 7-day click, 1-day view default. When a user clicks a Google ad on day 1, a Meta ad on day 5, and converts on day 8 — every platform claims the conversion. Your total reported conversions can exceed your actual conversions by 40–60% before you've made a single mistake.