They're done in alphabetical order of the .bdz files in the Boards directory. You can shift them around by renaming them.

The only exception is the "Hide and Glow Seek", which is never first. Since this set has a lot of invisible blocks, it could be a little infuriating to a new player, so they wanted to make sure it wasn't the first set listed. However, looking through the code, this only applies to the full version of that set, not the demo version.