Are they separate DVI and HDMI ports on the TV? If so, the TV broadcasts to the video card which resolutions are supported. When connected via DVI or VGA, most TV manufacturers consider this a PC input and they configure it to offer many resolutions. But when you switch to HDMI, the manufacturers consider this a Consumer Electronics connection, and usually stick to only 4 or 5 standard options (480i, 480p, 720p, 1080i, 1080p) and that’s it.
