The 744 is designed to only carry Center tank fuel when the main tanks are full. In this configuration the fuel in the wing tanks counter balances the wings' lifting force, enabling the center tank to carry a load which is burned off first before stressing the fuselage. If one of the main tank one or four pumps is u.s., you run the danger of uncovering the other pump during takeoff, so you need to maintain a minimum of 11,000 kg for takeoff per tank, which is obviously wasteful if you have a short leg but need to take 44,000 kg fuel due to this restriction. A way around this is to use the operative center pumps to feed engines one and four for takeoff by putting at least 7700 kg in the center tank, ensuring that the center tank feeds the outboard engines. This results, however, in a non-standard fuel distribution; i.e. center tank fuel loaded without the main tanks being full. So, at maximum zero fuel weight, you would have the wings producing a lot of lift, center tank weight exacerbating that effect, and less fuel than usual in the main tanks to counterbalance that force. Reducing the maximum zero fuel weight by the weight of the center tank fuel ensures that the structure will not become overstressed because the wings will be producing less lift, which then can be balanced by the lower than usual weight in the main tanks.