But why depict this on animus when their eyes are shown to be flat when viewed from angles? I think the actual reason is to prevent characters' expressions appearing to change as they look around.
this part is the widestThis part is the widestThe cornea over the iris/pupil bulges out from the eyeball. This bulge presses up the eyelid, so if the pupil looks to the left, the opening of the eyelid also moves left. This image illustrates that the direction of the gaze doesn't affect only the pupil, but the entire eye.iris/pupilsclera