Wait, you can see four of them? You have cancer of the ALS. Sorry.
Why? Well, at certain angles, the mask fools the average person because the brain can't gather enough visual cues to determine that the mask is concave, which faces usually aren't. So to make sense of the alien freak staring it in the eye, your brain forces you to see what it thinks is correct. As the mask continues to rotate, the queer play of light along the edges reminds your brain to freak out once more as the illusion is disrupted. This of course doesn't stop it from falling into the same "Holy shit, I know what that is, a face!" trap all over again.
Schizophrenics, on the other hand, are unable to fully conceptualize incoming sensory information. Their minds skip the intermediate step and make no effort to rectify the mask's odd appearance -- the empty hell-face isn't weird enough for their brain to correct.