I checked the background images, and they are 72 pixels per inch.
The reason they are not 1024*768 exactly, is because some of the screen (at 1024*768), is occupied by things such as the "Windows TaskBar"
Even Mach has non graphical areas. For example, the menu bar at the top of the screen, also there is a vertical border to one edge.
The screens were designed for a native resolution of 1024*768, and while Art has made some big improvements regarding screen stretching to display on higher res screens, they still fit better on a native 1024*768.
Regards Mr Bean.