I recently asked my rep about that and here was the response: Due to the large volume of data a Placement Performance report can
include, there are several factors that can affect the data that you see
in the reports. If you notice a discrepancy between the Placement
Performance report and the content totals in your account statistics,
please consider the following:
1. The Placement Performance report can take more time to update than your account statistics. We recommend you run this report after 3 p.m. PST to get the most accurate metrics for the previous day.
2. Statistics from some properties that are a part of the Google content
network may not be included in the Placement Performance report.
3. Sites that have accrued a negligible number of impressions per day may not be included in your report.
