Things in a mirror are not actually backward. The fact that your brain processes things as "backward" is a trick your brain plays on itself, because it thinks that the only way you could possibly be seeing the backside of an object is if you were standing inside the mirror looking out.
this says it better than i did
Its literally a mindfuck. Your mind is fucking with you. It sees a mirror and says "that shit cant be right, ill fix it"
They are upside down, but only in Australia.
Your eyes are side to side ( I presume) rather than one on top of each other. If you lie sideways and look in a mirror then everything appears reversed top to bottom.