My impressions are low compared to the number of visitors I have
ProfitSence
Last Update a year ago
Our statistics might be different from yours as far as we only count impressions but not visitors or page views. We score impressions only when our ads are opened and seen by a visitor to your website.
In addition, keep in mind that many users have an AdBlock, so this might also affect your performance and revenue.
Check Google's answer: https://support.google.com/admanager/answer/2521337?hl=en