My monitor does this if its scaling options are enabled. It treats everything that is not 1080p as 4:3, including 720p.
If this is the problem, the solution is the following:
1. Go to the setup menu of your monitor.
2. Find its scaling options and disable them. Set to full size/stretch/whatever.
3. Open CCC/VECC.
4. Set your resolution to lover than native.
5. Go to My digital flat panels section and enable GPU scaling (maintain aspect ratio)
6. Set your resolution back to native.
Note: You need a digital connection (DVI, HDMI or DP) to have GPU scaling options. It won't work with standard VGA (D-SUB).
CPU: AMD Phenom II X4 810 @ 3120MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 835/1135MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 3.11 Pro x64