Two systems reporting the same campaign will produce different numbers. The disagreement is structural rather than a fault, and attempting to reconcile it exactly wastes considerable effort.
Each system counts a different event
An advertising platform records a click when the user leaves it. An analytics tool records a session when a script runs on the destination page.
Between those two moments the visitor can abandon the load, block the script, or arrive with the referring information stripped away.
The two numbers are therefore measuring different things, and the difference between them is a real quantity rather than an error.
Attribution rules differ by design
Advertising platforms credit conversions to their own exposures, including impressions that were never clicked, within windows they define.
Analytics tools typically credit the last non-direct source and use a different window entirely.
Run across several platforms, each claiming credit under its own rules, the total of platform-reported conversions routinely exceeds the number of actual sales.
Timing shifts the numbers between periods
Platforms usually date a conversion to the moment of the ad exposure. Analytics tools date it to the moment of the transaction.
For anything with a long consideration period, that difference moves conversions between weeks and months.
Two accurate systems can therefore report different totals for the same period while agreeing entirely about which events occurred.
Identity resolution accounts for the rest
Logged-in platforms follow a person across devices. Browser-based measurement follows a browser, and treats the same person on a phone and a laptop as two visitors.
Storage restrictions, tracking prevention and cleared browsers each break the chain, and each break creates an apparently new user.
The direction of the resulting error is consistent, which is why platform figures usually exceed site figures rather than varying randomly.
A consistent gap is therefore a sign the systems are working as designed, and a gap that suddenly changes size is the thing worth investigating.
Choosing a system of record
The productive resolution is to nominate one source for each decision and to stop treating agreement as a goal.
Business outcomes belong to the system closest to the transaction, since that is the one that can be tied to revenue actually received.
Platform figures remain the right tool for optimising within a platform, provided nobody adds them together across several and presents the total as sales.