You are correct!! I have all my frame covered when i SHOULDN'T!!! I should have black vertical borders on the left/right.
No, when i set up my game on any 16:9 aspect ratio (1920 x 1080, 1366 x 768, 1280 x 720 if they had it), i have all my frame covered as it should be. The only difference when you change resolution but keeping the same aspect ratio is the scaling of the image because you give more or less pixels to your monitor which has a fixed resolution and aspect ratio. So, in any other case when you give it to handle something different of its original aspect ratio (this is the case...i gave 1920 x 1200 to a monitor with fixed resolution 1920 x 1080), i should have black borders...instead i didn't have.
No, my monitor is not doing scaling of any kind.
EDIT:
Why did you change your answer? Was correct the first time...lol, wrong but correct.