At this point in time, the way Google Analytics is separating users on AMP and non-AMP pages can make it hard to get a full picture of user behavior across both page formats.
Google, in order to make the analysis easier, is updating the way it counts and reports site visitors to AMP and non-AMP pages in Google Analytics. This is being doe by unifying user IDs when someone visits a domain via AMP and non-AMP pages.
Since the change is unifying user across both page types, this means that there will be some reporting fluctuations. If you want to find out more about the implications of this update, read the full article on Marketing Land.