The way Live works, you can get away with anything that doesn't specifically violate any of their rules. Creative spelling, use of numbers in place of letters, etc. they've already blocked out the most obvious versions, but occasionally Gamertags, such as yours, get by because they don't meet any of the predetermined invalid combinations. That is, until someone reports you. Then it gets sent to an actual person who decides whether it stays or goes. Looks like that is what happened to you.