When viewing ad unit reports, you may notice a discrepancy between the total number of impressions for the top level-ad unit and the sum of all impressions in second-level ad units. The reason for this discrepancy is because some ad tags on the site only include the top-level ad unit but not any second-level ad units.
For example, reporting data for the top-level ad unit dfpnews.com shows that it served 200,000 impressions in a day, but when you click into that ad unit in the report and view the second-level ad units, their impressions add up to 185,000. The missing 15,000 impressions come from ad tags that only contain dfpnews.com and none of the second-level ad units.