7 Star! The last in the set... if you haven't collected the previous six then what are you waiting for? Gotta Grab 'Em All! Need a selling point for a single 7 Star Crystal Ball I suppose?

Ever hear of Ben & Ron? They were a crazy pair of interstellar dragoons who were brought to life by poorly thought wishes by people on the planet Manek. Together, Ben & Ron shared a single crystal ball, the 7 Star! They couldn't ever decide on how to share the ball between the two of them, so someone named Yajirubberu sliced it off one of them while they were busy fighting each other. Then he delivered it to Kammy, the OP goddess of Earth 5 who was also a Manekian, who then created the other 6 you've seen us selling.

So the 7th was the original, so that should be a good enough reason for you. Some people like to argue this story is not canonical but whatev, totally true.