Normal operation. When you are lowering the resolution, you are most likely also choosing a different display aspect ratio. You can tell be dividing the width by the height.
1920/1080 = ~1.77 - ~1.77 is what your monitor likes = 16:9 aspect ratio.
If you choose for instance, 1680x1050, that's 1.6 = 16:10 aspect ratio.
The difference in aspect ratio of 1.77 and 1.6 will cause the black bars on the screen.
In the driver, there may be some function to stretch the resolution out to make it full screen, but this is not what you really want to do.
Leave it as 1920x1080 and enjoy a nice screen.