The title update(number 2) for Halo 3 is released on 9/25. M$ made it official that any DLC costing a certain amount for a title could add 250 achievement points to a game. Bungie has released 2 map packs and there is apparently another one coming. This is where the 750 achievement points are coming from. Halo Mythic was just the build of the game that was at PAX.