For me, I think it's that the movement of the rest of the mouth and the muscles/skin in the face (and the light/shadow as a result) doesn't match up with what I

For me, I think it's that the movement of the rest of the mouth and the muscles/skin in the face (and the light/shadow as a result) doesn't match up with what I expect. It looks like the facial animation from a good videogame cutscene