so i guess you do not care what the game looks like? i think since windows vista they don't let you use a resolution that low. especially if it is not the recommended resolution for your display. whatever game you are playing may allow you to set it lower
Some newer games have a scaling option. So if you set it at (say) 50% and your screen resolution is 1920x1080, the game would render at 960x540, then rescale (stretch) to fit your 1080p display. There's no consistent setting for these though. I've seen both a percent scaling and render resolution (as opposed to display resolution) used in the options.