Is it just me, or do games that are established as good or well selling seem to jump into the thought that they need to add multiplayer? How can you assassinate when your target(s) know you're on the way?
But going to the point of the total of sales, I think it's because people liked AC1, but the grind was a huge problem, so they had to see if AC2 fixed it, which it did (apperantly, haven't played AC1 but have started AC2 and am enjoying all it offers) It seems to me that if a game is the first in a series, unless it gets a LOT of hype from the media and from critics, it usually gains power slowly. But tag a number onto a known game and it sells extremely quick, no matter how good or bad the game is. How high they can go with numbers depends on the quality of the game and the level of their fanbase.
But from what AC2 did compared to what I've heard about the upgrades from AC1, it seems that Ubisoft is doing well in the "fix-our-mistakes" section by removing repetitiveness and adding a lot of things.