When it was developing the upcoming Mario game, the character's creator Shigeru Miyamoto says that the game had a *slightly* different temporary title:
Super Mario Galaxy 1.5.
But as Nintendo was making the game, the developers kept coming up with new and interesting level designs and ways to play. The game features new spins on the franchise with the likes of Cloud Mario and Rock Mario. With over 90 percent of of the level design being brand new, Super Mario Galaxy 1.5 naturally became Super Mario Galaxy 2.