I use rFactor 2 below my monitor's native resolution, and in the DX11 version when I use full screen mode, the size of the picture changes but the monitor resolution mode doesn't. So I have a smaller, borderless "window" in the corner, and I still can see the rest of the desktop in the native resolution. I'm in full screen mode for sure.
My fullscreen doesn't work even if its the same resolution as my desktop. Pretty sure fullscreen isnt working full stop. You can tell when you alt + tab the game doesnt minimise.