I think they're trained to call anything even remotely close as a fumble. That way the play can continue the way it would if it were a fumble, then since replay is in effect on all turnovers, it will be simply be overturned if it wasn't an empty hand, was down by contact, etc. It works as a default position because it avoids them running into a situation where they whistled a play dead and replay shows it should have still been a live ball, and then you just have a big mess on your hands.