Tips for Atomx Publishers -> Reporting -> Blocked Impressions:

“Why do publisher statistics differ between Atomx and mine?” Differences between stats are common, and usually gaps between 1 and 5% are acceptable. 10% would already be a matter of concern. If you see significant discrepancies between Atomx and your external reports, The reason will most likely be bot traffic and/or fake traffic. Using a sophisticated fraud filter technology, Atomx blocks bot traffic (e.g. Google index bots, search engine bots, fake user agents or browsers, traffic from suspicious IPs). Our anti-fraud algorithms automatically detect and block suspicious traffic before it reaches the bidding stage. Sell side statistics show this traffic as ‘Blocked Impressions’.

Atomx Fraud Filter explained: We do pre-bid and post-bid calculations based on data of incoming traffic. This fraud filtering happens on IP level, on user level and on browser level, and will cause bot impressions to appear in 3rd party statistics. No advertiser will have to pay for this traffic. We hide these bot impression within Atomx stats.

Different time zones. If your statistics show numbers by a time zone other than Atomx (UTC, EST, PST or CET), the two sets of reports will be aggregating different time periods for the same displayed date. That might also be a reason for gaps between stats.