I think it has a lot to do, partly at least, with the legs Nintendo games tend to have. See, after seeing the first ten weeks of a 360/PS3 title, things usually don't change that much. Of course, there are exceptions (Looking at you, Valkyria Chronicles), whereas with Nintendo games it can go pretty much any which way. I know that I still regularly check the Mario Kart Wii page and am still amazed every single time at how much it's grown from the newest numbers.