i think a lot of the time reference images can be off, because of the lens used and distance from the camera. if the subject is close to the camera there can be warping.

maybe try duplicating the head have the two side by side. Then just push the corner of the mouth in a bit with soft selection on, see if it looks better, you could use the sculpt geometry tool on smooth a tiny bit to average out the mesh in places.

soft selection shortcut is B on the keyboard, hold B and the middle mouse button to increase and decrease the fall off.