We count impressions only when our Onclick pop-under ads are opened. Thus, there are several reasons for the difference:
Some of your visitors are using ad-blocking plugins in their browsers or other ad-blocking software.
Solution: Ask your visitors to add your website to their white list.
Our pop-under ad channel code is not placed on some pages.
Solution: Check for the pop-under ad channel code on all your pages.
Some mobile devices don’t serve the pop-under ads.
Solution: Use the mobile-optimized ad formats to maximize coverage across devices.