Now about numbers. It is estimated that the amount of Linux players for Quake3Arena was about 20,000 and that was almost ten years before when the Linux market was much smaller than today So if we say that a new AAA title realeased simultaneously for Linux and windows and sold lets say 500,000 copies, then 25,000 are sold to Linux users. So lets say 50$ per box, we have 1,250,000$. Do you find this number insignificant? Not at all if we consider that to port the game needs much much less money than this. And again, the cost for advertising an AAA title for Linux is much smaller in compariosn with the windows equivalent. Actually, it doesn't need advertising at all. It's enough to just say we will port this AA game to Linux and the Linux magazines, forums and sites will be on fire for free.
Also, don't forget that there is a significant part of window's users (maybe the majority) that do not care about computers and use windows because they just found PCs with them inside and the only they do with their computers is some casual jobs. Also, another part are the ones that need some proffesional application for their job etc. All these reduce the windows population who like to play games from 90% to the half. On the other hand the Linux users who choose Linux as their desktops are people who know few things about computers. But to know few things you have to love computer even a bit and for people like them, gaming is easier. Because of that we see that games sell 5%+ in Linux while Linux has lesser market share.