Short answer: Up to about 10% is usually treated as normal. Different systems count impressions at slightly different moments, so some gap is expected. Larger or growing gaps need a closer look.
Why numbers never match exactly
- Counting point: one system counts when the ad is requested or delivered, another when the creative renders.
- Timeouts and slow creatives: the SSP may win the auction, but the ad never loads before the user leaves.
- Filtering: each company filters invalid traffic in its own way.
- Time zones and currencies: reports cut days at different times and convert currency differently.
How to investigate a big gap
- Compare the same day, time zone and currency in both reports.
- Check the gap per ad unit and per device. A single broken placement often explains most of it.
- Look for slow or heavy creatives and for ads that load below the fold.
- Ask the SSP for their impression and render counts for the same period.
- If the gap stays high, lower that partner’s bid adjustment or test pausing it.
Related: ad tech glossary: discrepancy and where to place ads.
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