I just merged this fix which means that transitions between similar pages (e.g. topic → topic, or topic-list → topic-list) will now correctly reload any ad units on the page. That should lead to a slight increase in impressions - the precise amount will depend on usage patterns.
I can’t find any information from Google about how they calculate the pageview metrics for Adsense.
Discourse is a Single Page Application (SPA) which means that navigations around the forum are not ‘real’ pageviews. Google Analytics and Discourse’s own metrics understand this, and track the fake pageviews.
It’s possible that Adsense only counts ‘real’ pageviews. If so, then that would explain the lower numbers when compared to GA/Discourse.