Ad serving discrepancy refers to a difference between the number of ad impressions counted by a publisher or ad network adserver and the one counted by the agency or advertiser ad server.
Ad serving discrepancy can be the source of billing issues. To account for the discrepancies and prevent underdelivery, many publishers are padding their impression goals on their own ad server.
Ad serving discrepancy is lessening with adoption of IAB standards and guidelines, but often estimated to be between 5 % and 10 %.
See also impression discrepancy.