GA4 and advertising platforms disagree because they observe different events, identify people differently and apply different attribution rules. The goal is not to force identical totals; it is to understand each system well enough to make a defensible decision.
Check the event before the attribution model
Confirm that the same business action is being counted, with consistent value, currency, deduplication and order identifiers. A tracking mismatch cannot be repaired by changing the reporting window.
Account for identity and consent
Browsers, devices, login state, cookie restrictions, consent choices and modelled conversions change what each platform can see. Platform systems also have an incentive to report conversions influenced by their media.
Assign each source a decision role
Use commerce or CRM data for confirmed outcomes, analytics for on-site behaviour and cross-channel patterns, and platform reporting for delivery and optimisation signals. Reconcile them in a repeatable view.
The decision to make next
Document the event definition, attribution window and expected variance; investigate changes outside that tolerance instead of treating one dashboard as absolute truth.