I've heard this complaint before; how can the game run at a higher resolution than your screen? And does it actually look better that way?
It just resizes the resolution when you boot up the game (I play fullscreen). My desktop's resolution is actually lower than what my monitor can handle, since I'm not comfortable with small icons and fonts etc.
I just want to play my games at a higher resolution at 1920 x 1080 which is my monitor's preferred resolution (any other resolution will be a little blurrier than the preferred resolution). When you exit the game, it'll revert back to your desktop's resolution.
What your computer can handle and prefers to are dependent on the graphics card and monitor.
Back on topic, I'm not sure whether Valve found this out yet (seems not as it's not fixed after two patches).
Use the launch options -w and -h (I think you can guess what they do.)
Oh, I tried that definitely. The game doesn't care, it just goes back to that 'native' resolution. I semi-fixed that by making an autoexec.cfg file that resizes the resolution on startup.