It certainly wasn't! It was a flat map with an hexagonal grid which covers an area roughly between 15 to 55 degrees latitude - so as you'd expect the distances east-west across the map are actually shorter the further north you go.

The Oerth Journals (free for download - can I post websites on these forums?) numbers 3 and 4 go into detail about how this effects things much more eloquently than what I could of.