I have a completely inane question.
The wallpaper is 1920x1223 rather than the usual 1920x1280, which means that an area of 1920x57 pixels was excluded.
What was in that excluded area?
The truth is that it's just the size it needs to be for the page, and the reason it's 1920px wide has little to do with wallpaper size. 1920px is the best balance between common browser size and download filesize, and the height is just the same ratio as the original mockup referenced used when I built the page.