Stats don’t just suddenly change if you didn’t update/change AdRotate.
If the code stays the same, the routine stays the same.
It’s possible that because of caching it never counted right and now it does, or that something else is doing something weird with the site – for example loading pages over and over. This could be a bot that’s not counted by your analytics.
But otherwise, I don’t see how this would change if AdRotate didn’t change.
Advert impressions are not pageviews. They should not be compared. But similar trends can be observed ofcourse.
More pageviews will (or should) result in more advert impressions.
Where Google may count per session, unique visitor, a gross number or whatever they do these days.
AdRotate goes for a semi-unique number. When the advert is loaded an impression is counted. Then a timer starts running. If the advert is reloaded before the timer runs out, no new impression is counted. If the advert is reloaded after the timer runs out, a new impression is counted.
You can configure the impression timer in Settings > Statistics.
Another influence may be bots and crawlers. Google doesn’t count them. AdRotate tries to not to. But you may want to review and edit the bot filter with more keywords. Which you can do in Settings > General.