Mirror Weirdness

If you're like me and can't stand it when somebody asks you to name all seven of the seven dwarfs, or answer a question that appears, on the surface to be easy, but in actuality, is just plain bizzare, then you won't like this.

How come the image in a mirror if back to front left-right, but not upside-down?

I looked it up, then got even more confused.