So you can "distribute objects" or "Distribute Spacing". In my example, I used "Distribute Spacing" - which puts the end boxes on the edge of the artboard. It sound like you want to distribute the objects (not the spacing). This (Distribute Objects) will create equal spacing between them, but not bring the objects out the edge of the artboard. You want to distribute the spacing evenly, but have them centered on the artboard?
Edit - I guess you have to place the outermost object to the edge to distribute them across the artboard.