You're almost correct. FFvsXIII simply has not been announced for the 360, yet. There was a time when people thought FFXIII was a PS3 exclusive as well. But as it neared release, they made a big deal out of revealing that it was coming to the 360 just before
the closing of Microsoft's press conference at E3. I'm sure they will do the same thing with FFvsXIII. The only reason FFXIV is a PS3 exclusive is because Microsoft hates MMO's and they turned it away.