Probably sidewall stiffness. With the rear weight of the car pressing on just the inner portion of the tire (and ultimately the inner sidewall), I'd imagine that the tire would deform/comress more than a more evenly distributed weight.

Or. It's a visual trick. The car is the same height, but you've rotated the tire edge down from the fender. Blah blah pure geometry, etc.

Or. your tire is flat. pump that bitch back up.