I can read upside down, but it's an acquired skill. I can't read at a 90 degree angle without difficulty, although I could probably learn to.

It's not that I don't see the map from the other side, it's that when it's the right way up I see all the extra information I have about it. For example, I bet an eye tracker would show me focusing on Western Europe, Central Asia, Australia, and the US. When the map is flipped, I see it closer to how it really is because I can ignore those preconceived ideas more easily. I don't see e.g. the Iberian peninsula as represented by a land mass, I see the actual land mass, and can concentrate on its size and distance more easily.

This is really interesting!