RTS, Halo: Wars, wasn't made by them. It was another studio and they had no control over it. Microsoft controls the I.P.
ODST was a simple expansion to Halo 3. Most games throw out expansions a few months after release but I have to admit it was overpriced which probably made it seem like a new game.
Halo 3 was the last one. This is a prequel to the whole trilogy, set in the same universe and featuring an entirely new cast of characters.
PS. I didn't know you had a 360
see that I understand, I wasnt being mean before I really thought 3 was supposed to be the last game of the Halo franchise
Man the Beta for this was incredible fun... and I definitely can't wait for the single player. It's hard to believe it arrives in less than two months... I'm just gonna have to flunk my A-Levels so I get enough time to play all these games...