In the current state when all of a games achievements are finished and they release a new DLC they can add new achievements and they change the completion % on games that were "perfected" before. Separating the achievements for the main game and DLCs would keep the "perfection" stats the way they are unless its something like Early Access Games. Also it gives people the ability to "perfect" a game they own without having to own all the DLCs. And you have a chance to add another kind of completionist showcase for DLCs to the profile (and the point shop)