I now realize why the wallpapers are wide like this ... the new Dock app will change the customized wallpaper depending on orientation of the phone: portrait or landscape. The built-in wallpapers are all this 9:8 aspect ratio, and only crop during selection for home screen wallpaper. When you select one for Dock, and rotate the screen, you're given a 16:9 crop instead of 9:16.
I presume the same occurs with Sense 4 wallpaper when you switch to landscape orientation? DANG I wish Sense 3.6 had that feature.