Pretty easy.
The camera *isn't* directly behind the actor. If it was, you'd have the actor's head in the middle of the shot (which would make for a boring shot). The camera is usually offset just enough to the side. The actor, in such a shot, is not seeing him/herself in the mirror, but the camera.
The only straight on shots I can think of off the top of my head has the camera looking out of the mirror, not from behind the actor (ie, you see the front of the actor's face only).
JHutch