1. Multiple tags firing
Channel app, manual gtag and GA4 import all reporting one purchase. The most common cause by a distance.
2. Attribution window
Google credits a sale to a click from weeks ago. Not wrong exactly, but it means the ad did not necessarily cause the order.
3. View-through conversions
Counted when someone saw an ad and did not click. Useful context, terrible thing to bid on if you did not realise it was included.
4. Modelled conversions
Google estimates conversions it cannot observe. Real methodology, but it is an estimate, and it does not appear in your bank account as an estimate.
5. Cross-device
Phone click, desktop purchase. Legitimately attributed, easily double-counted if another source also claims it.
The rule
Reconcile to the store. Use platform numbers to steer the bidding, and the store numbers to decide whether the business is working.