Eye placement is irrelevant - just turn your head to the side and you'll see your eye's being vertically displaced doesn't change the reflection. Basically, you're imprinting your image on a surface and it's being turned around to show you, like flipping a page. It doesn't go up and down because those are not relative directions. Up is up and down is down, but left and right are relative to the direction you're facing and the mirror is facing the opposite direction.