opinion: the overlay should have added borders to cover the matting completely.
. Layers & Masks both work the same way however, many times people forget the basics:
if we center the girl with the microphone in the picture then everything's fine - right?
. but how much larger would her overlay need to be if we moved her up, down, right or left of center;
what we are trying to cover-up would bleed through and destroy the clean background behind her!
. You've already isolated the girl with the microphone from her background and made everything but
her "transparent" so how hard is it to remember to add More transparent size to "her background" so
you can move her around the stage ( if need-be )
Editing is the simple Art Of Illusion, even Animators do things in Layers ^__^