Impression discrepancy refers commonly to discrepancies in reporting ad impressions between publishers, ad exchanges, advertisers/agencies, advertising networks and third party vendors.
For a single advertising campaigns several actors may be counting impressions each on his part. Impression discrepancies may sometimes rise billing issues and disputes.
Impression discrepancies are commonly due to the use of different counting methodologies, the fact that each actor has a different place in the delivery chain and sometimes to difference in defining what is an ad impression.
For instance, if an actor uses pixel based counting and an other the 302 redirect method, there are usually some differences.
IAB has edicted standards and guidelines to limit impression discrepancies between publisher, adnetwork/ ad exchange and agencies.