What X driver are you using? I seem to remember that the vesa/VGA drivers only supported 4:3 ratios or something like that, which might explain this behavior. If you're using Intel or similar, it might be that your EDID info is not being reported properly (or something doesn't know how to read it correctly. I can't really say). This[1] explains how to add undetected resolutions. Google also had much more info.

1: https://wiki.archlinux.org/index.php/xrandr#Adding_undetected_resolutions