The 1920 x 1080 resolution display is a good match for 720 x 400 resolution video to maintain the aspect ratio because:
- 1920 divided by 720 = 2.666666666667...
- 1080 divided by 400 = 2.7
Since the results of that quick math yield such close results, any black bars that would normally show up to fill the void on different resolutions will likely be so thin that they'll just blend in with the outside of the viewable screen area.
What I suspect is happening is that your video player software is increasing the size of your video by 2.666666666667 pixels (or something very close to this) on both the horizontal and vertical aspects, and just leaving the small difference black, which is not really noticeable (and certainly less noticeable than stretching to fit the entire screen without maintaining the aspect ratio because then some things will look oddly, yet very subtly stretched).
An easy way to test this out is to switch out of full screen mode with your video player software, and see if the thin black edges do show up between the GUI borders of the video player window and the video itself. Usually Alt-Enter is the keystroke to use to switch in and out of full screen mode -- if not with your software, then try VLC Player as I believe it does have this feature:
VLC Media Player (free, and open source)