Yep, Ziz is right. In Lucas's original twelve movie plan, each film would tackle another aspect of the universe, under different directors, following different characters, and not necessarily in chronological order, even though the movies would still be numbered. The shooting script for Star Wars was titled: "The Adventures of Luke Starkiller As Told in The Journal of the Whills Chapter One The Star Wars" or something similar. It just wouldn't have mattered what the numbers were in term of chronology. It would have been similar to The Chronicles of Narnia before they went back and f'ed up the numbering of that series.

However, as to your question, Jenny. I'm glad those restrictive episode numberings are there as it provides something of a lock to keep from making any more movies and ruining Star Wars even further. I mean, even that doesn't matter, as The Clone Wars proves and both the new TV series.