What complicates matters is there's no good way of knowing how many Steam "Windows" users are people who use OS X or Linux as their main OS but boot into Windows to use Steam because of poor graphics card drivers.
A very related group runs Steam in WINE so they can launch and play Windows-only games. That must skew numbers significantly as well, and unless you only play TF2, Linux users are probably being counted as both Linux and Windows users because they play one game in Linux and another in WINE.
