Dawntrail Benchmark Resolution/screensize

The benchmark defaulted to 4k settings of 3840x2160 and while it ran fine, it was too large. This setting can only be achieved in Windowed mode where it offers CUSTOM resolution settings. The bottom of this image is cut off because its too big, i have 2 monitors so i was able to capture the full window's width at least, and look at all that game, edge to edge the window is full of beautiful game.

When i put my settings back to the normal 3440x1440, edge to edge is black borders and all that real estate is going unused.. WHY?!?