Because they count different things, on different clocks, with different rules: Ads credits a conversion to the click date and its own attribution window; GA4 logs it on the event date with its own model. Clicks vs sessions differ too (one person, one session, three ad clicks is 3 vs 1).
Add consent-blocked tracking, ad blockers, and modeled data filling gaps differently on each side, and 10–20% divergence is normal operation, not breakage.
The sane practice: pick one system as the decision ledger per purpose (Ads for bid optimization, GA4/CRM for business truth), compare trends rather than absolutes, and chase only large or sudden gaps — those usually mean a broken tag rather than philosophy.