All values are not in fact constant. Exclusive titles carry much more weight early in a generation then they do later in a generation. Microsoft had more exclusives when it mattered, and that is a major reason why the 360 has had more sales then the PS3 in the market. The point is this consumers generally do not care today. Whether the game that is on the shelf is exclusive to their platform or not. They will buy the same number of games even if the proportion is heavily in favor of multiple platform games.
Gears of War 3 is actually performing as would be expected, and you need look no further then the fact that is is the premier fall shooter on the 360. Which always sees disproportionate sales. Every year the 360 has some game that simply prints money, and this year it is Gears. This happened when the 360 had a shit load of exclusives. So if that is your theory your simply clueless. Gears is probably going to end up selling over ten million units in its lifetime. So it goes without saying you should expect to see a third to half of those sales in the first week or two on the market.
Before anyone says oh well that is opinion. No it is just a good rule of thumb for sales. A third to a half up front is just the way almost every single game works out. Unless a bigger title stifles the sales at launch time. Seriously is it something like the charts have a communicable disease. Why not use them to get some insight.







