Comment 9 for bug 684982

Some window managers do multiple desktops by having one large space that windows can be moved across. (In such window managers, you can generally have a window that appears partially on one desktop and partially on another.) In these window managers the desktop of the window is stored simply by its coordinates in the large space, and we are just restoring those large coordinates, since we restore the window's position.

(However, this can cause problems if you ever change screen resolution. Some window managers, like compiz, don't even handle fixing the position of windows currently open when doing changing screen resolution. But the window manager couldn't simply do it automatically with session restore.)