As I imagine it, the maps do not actually encompass the entire terrain of a planet, here are some reasons why:
1. For instance, I was under the impression that the eastern edge of the Correlia map was not meant to lie touching the western edge. If that were the case, then the bodies of water clearly do not line up properly.
2. Also, if the area available for travel constituted the entire planet, then every planet is exactly 16km in circumference.
3. Finally, if you anticipate that the farmost edges of the x-axis actually lie on a single line, then the y-axis clearly can not adhere to the same rule (or vice versa) because the "corners" of each world would overlap. Spherical geometry is tricky like that.
I don't know what the official word is on this issue. I'm sure that some serious SW fans could claim grevious error in the planetary geography. In fact, I think I read some complaints about how Tatooine is arranged. Unofficially, I like to think that there is more to explore beyond the invisible square cage, and that later additions to the game might some day expand the planetary maps.