Thats technicaly not the way xyz works. The Argument xyz selects Enties based on their distance from that position. If you specify no distance (Radius, or as you said, Volume) all entities will get selected. If you specify a Volume dx,dy,dz or a radius r it only selects Entities in a radius/volumen from that position.
Edit: Sry, read your reply again, it seems you said just that *facepalm*