Well, it flips front-to-back. But this side of the mirror, we can't physically do that, so it's not in our brain's list of expected possibilities. We can, however, rotate ourselves 180 degrees. And because we're bilaterally symmetrical, our brain finds it easy to interpret the front-to-back flip as a 180 degree rotation. But that leaves left-to-right weird, because what happened wasn't actually a 180 degree rotation.