If you have a display with a big resolution, and want to run it in "half" resolution, you'll end up with blurred image because of the scaling filters on the Nvidia Windows drivers.

For example, the MacBook Pro retina 15" has a huge 2880x1800 resolution, and it would make much sense to run it in 1440x900 for games. The bad news is that image will look blurry, even though it would be possible to render image as crisp as on a native 1440x900 display ("pixel doubling" without any fancy algorithms to blur the pixels).

Some related discussion:
http://forums.macrumors.com/showthread.php?t=1389054&page=1
http://apple.stackexchange.com/questions/59283/why-is-a-retina-macbook-running-1440x900-grainy-under-boot-camp

if (target_width != width / 2 || target_height != height / 2) {

    fancy_scaling();

} else {

    dumb_pixel_doubling_scaling();

}