When your numbers and theirs will never match

When your numbers and theirs will never match
Discrepancy is normal. Unexplained discrepancy is a problem. Learn the difference or you will either accept theft or start fights you cannot win.
Legitimate causes:
— Different counting points. You count a click leaving, they count a page loaded. Everything lost in between is real and unavoidable.
— Different time zones and different day boundaries.
— Deduplication rules. Repeat visitors counted once on one side and twice on the other.
— Filtering on their end for suspected invalid traffic.
— Delayed conversions landing in a later period on their side.
What to do:
— Agree in writing which system is the source of truth for payment, before you start.
— Track the discrepancy as a trend rather than a single figure. A stable gap is a counting difference. A widening gap is a conversation.
— Keep your own logs detailed enough to reconstruct a day if asked.
A gap you can explain is arithmetic. A gap nobody will explain is a decision.