Uhm, it should.
The native resolution of 1920x1200 is what the monitor is meant to be used at. Taking off 120 pixels would either stretch the image up (which looks horrible) or cause those black bars to be in place of the pixels not being rendered.
This is natural; 1920 x1200 is a 16:10 ratio, while 1920x1080 is a 16:9 ratio.