AdSense is changing the way it measures impressions by introducing downloaded impressions based on recommendations from The Interactive Advertising Bureau (IAB) and Media Ratings Council (MRC).
So far in AdSense reporting, an impression was counted when an ad matching the request was located on the server, this was called the “served impression”. Going forward, impressions will only be counted when at least one of the ads from a request starts to load on the user’s device, this measurement is hence called a “downloaded impression”. From May 3, this is the metric that publishers will see in their reporting panel, change is in line with the push for increased viewability in reporting metrics.
Switching to counting impressions on download helps to improve viewability rates, consistency across measurement providers, and aligns the impression metric better with advertiser value. For example, if an ad failed to download or if the user closed the tab before it arrived, it will no longer be counted as an impression.
Source: Inside AdSense blog
Google is expecting publishers to notice a drop in their impression count, with a corresponding improvement in impression RPM, CTR, and other impression-based metrics. In the official statement about the change, Google has mentioned that publisher earnings should not be affected due to this change.