Why don’t my ad impression numbers match between platforms?
Ad numbers rarely match because each platform counts at a different moment and filters differently. An ad server may count when an ad is requested, another when it begins to render, and a verification vendor only after invalid traffic is removed. Timeouts, ad blockers, time zones and bot filtering also cause gaps; up to about 10% is often treated as normal.
A discrepancy between, say, a publisher ad server and an advertiser ad server usually comes from counting methodology (begin-to-render versus downloaded), latency (the user leaves before the ad loads), different GIVT filtering, and reporting time zones. Many contracts name a billing source and an acceptable discrepancy threshold.
Persistent or growing gaps deserve investigation: they can reveal hidden ads, redirect chains, tracking errors or invalid traffic that one side filters and the other does not.