Windows 8 has a minimum resolution of 1024x768, unless you are running it in a VM. As such it doesn't support going into 1280x720 (since 720 < 768). Windows 7 allowed this, but the modes are missing from Windows 8.
My video card and monitor support 1280x720
But it is missing from the slider:
How do I force windows 8 into 1280x720 ?