A: The user may have closed the page before segmentation occurred

This article explains the difference between the predicted audience size and the live audience size of a segment.

The predicted audience size counts raw Pageview events whereas the live audience size counts the cohort entry events.

The Pageview event is the first event to fire on the page. If a user closes the page after the Pageview event fires but before the cohort entry event fires, this could result in the predicted audience size being higher than the live audience size. 

This could also happen in AMP as, while events are tracked for all users, segmentation only happens on the second pageview. 

Note: The predicted audience size will be larger than the live audience size if the segment is less than 30 days old.