Monitor's native res is 1680x1050. 1680 divided by 1050 = 1.6 = 16:10 aspect ratio
Choosing any non 16:10 aspect ratio will correctly leave black borders. Nothing is wrong and function is as supposed to be.
If you want to play games without borders, always choose a 16:10 resolution.
If you use other resolutions, you will have to stretch the screen out (possibly with driver
app) and that will cause a distortion in proper aspect ratio. A square will not be square, rather, rectangle.
Advice is to play under resolutions above.
1280x720 is 16:9 ratio.
Some games, you can choose resolution and also the aspect ratio you want to use. In this case, always choose 16:10.