I confess to being confused, loaded your file and see exactly what you mean. So, I added a new page, two columns, used 1920px image as the background with the same 1920 image in foreground then viewed in browser on a 5k monitor full screen and it centred for me! The only thing I had to do was set padding to fullscreen and that only to stop the image being cropped off at the top and bottom and nothing to do with the issue at hand.
I also spotted that the footer detail does use the full screen on your page, even though the nav and body doesn’t. That said on the page I added the Nav did use the full page, along with the body too.