No, that thing what you describe is stretching smaller display size to nominal LCD resolution. In my Dell this option is called "Wide mode" and is set to "Fill" by default.
I've asked the same question at Dell forums and will see who's gonna to confirm first that is "known" issue of its brand ;-) So far Dell's admin deleted my post once, huh.. and what about BIOS emulator in GB graphic card, is it true that VBE doesn't support FullHD resolution, just 1600x1200 ?
Maybe it's possible to change some values using Nibitor then ?
P.S.
My mobo is Asus Striker II Extreme, but even if I customised somehow PC's BIOS (to avoid borders during POST & BIOS settings), how to customise my OS' bootsplash & GRUB2 background, which is set to 1920x1080 and looks like FullHD, but a bit downscalled + borders added ? BTW, 1680x1050 is 16:10, not 4:3-based.