Well, powerman seems to have left the building, so I'll give it a go. The skimmer tube is a fixed height (T), but the pump does not need to support the water within the tube, from the waterline of the sump down (D). This water is supported by atmospheric pressure. The pump only needs to support the water height above the water line of the sump (T-D=H). Therefore, if the skimmer is sitting deeper in the sump, it does not need to support as much water and can move more water as H is smaller. If it moves more water, it is doing more work in a given time and therefore using more power, as power is the rate which work is performed.