Dog star,
Think of the effect of fuelling the aircraft. As the fuel fills in the wings(747-400)The fuel effect takes the fuel vector along way forward up to 125,000kg. After this the tail tank comes into play and as it fills at a ratio of 1kgaft/9fwd approx., the trim effect moves aft.
So much so that at MTOW 397210kg approx., if a/c is too tail heavy at ZFW a fuel change of 100kg in the tail tank will put a/c out of trim at TOW.
The shape of the lines reflects the way the fuel travels/fills the wings during fuelling. Except for non standard fuelling scenarios.