Actually, world -1 is world 36 (or something) of the game's files, and many unused worlds use levels from the original 8. Some levels where actually never used in the first 8 (somebody discovered it in a storm related thing, and thus led to the belief of a world 9 (which the idea of a secret world[s]/world 9 was used ever since). Also, there are 256 worlds in the game (appearently), all unused.