This is a great question that comes up all the time from publishers. At a glance, it seems like the number of pageviews should be directly proportional to the number of ads.
Suppose you’ve got 1,000,000 pageviews. Every page has 3 ads. You should see 3,000,000 ad impressions in DFP, right?
Well, it’s actually a little more complicated.
There are three big reasons for the difference and a bunch of small ones.
Second, anywhere between 10% – 40% of your readers are probably using an ad blocker. Google Analytics will count these as pageviews, but your ad server won’t count them as impressions.
Third, you probably don’t have exactly 3 ads per page (or maybe you do!). It’s likely that you have different sections of your site. Some pages have 2 ads, some pages have 4, and some have none. You might have no ads at all on your mobile site. Any of these scenarios would throw off a simple calculation of “number of ads per page”.
Despite the difficulties, “impressions per pageview” remains a useful metric to track. A big jump or decline in this number likely means something is very wrong.
If you want more details on reporting discrepancies between DFP and Google Analytics, be sure to check out the official reasons stated here: https://support.google.com/dfp_premium/answer/177405?hl=en