I think they all must have a hidden ID tag (just a number) and they're sorted based on that. The actual order might arise form when they settled on the name (since it doesn't seem to be category, implementation, alphabetical, etc.) and entered it into the database. I've noticed the same thing with the minion guide window. Minions are sorted in a predetermined order that is not based on when they were released (notice Midgardsormr on the front page). Of course in the case of Midgardsormr, that minion may have been created or at least assigned a tag ages ago when they first knew that they would add him.