Windows not filling up screen

Hi, I just wanted to know a simple thing related to VGA cards

Is Windows not filling up the entire screen normally a resolution problem or a sign of a failing VGA card?

What I mean is, say I open Internet Explorer or Windows Explorer or even Microsoft Word. There is a gap in the bottom between the Taskbar and whatever it is I have opened. The problem is not there if I press the DETECT button in the Screen Resolution menu.

Just curious.

Thanks!