Performance report discrepancies
Ad Exchange Placement Performance reports show sites that have achieved either more than 100 impressions, 1 click, or 1 conversion; Campaign/AdGroup Performance reports show an aggregation of all impressions. Because of this difference, you might find that your Campaign Performance and AdGroup Performance reports do not match your Placement Performance reports. Campaign/AdGroup Performance reports can sometimes report up to 40% more impressions than what is reported at the site level (i.e., in your Placement Performance reports).
When you start creating campaigns, it's best to set up a run-of-network ad group to target every vertical category. After a few days of running, we recommend evaluating the sites the ads served on by running a Placement performance report. Use the report to determine the sites that have performed well and target them explicitly. This means that you remove category targeting in favor of targeting sites directly.
To find additional opportunities, enter the well-performing sites into the Placement tool to find other sites to target.