Yeah, you have vertical sync enabled. What it does is that it syncronizes your card's ability to render a frame with each refresh of your monitor.
So, if you're running at 1600x1200 with a refresh of 85hz on a CRT and using vsync, you will only be outputting 85fps. If you're running 1024x768 with a 60hz refresh, you'll only be outputting 60fps.
Then, there's also the situation where if your video card does not have the power to sustain that framerate, your frames will be cut in half. There's a feature that helps mitigate this framerate cut, but I forget what it's called. So, if you were running into that sort of situation at 85hz, your FPS would become 42fps, and at 60hz, it would be 30fps.