OK, I should have asked "why does the mass of fuel required in the tail change?"
To give a specific example, the fuel load requested was 135,000 kgs with 3900 kgs in the tail. SG was 0.795. The engineer had written down a few other tail figures from the manual, e.g. at 0.790 3800kgs was required. I would have guessed that as the SG goes down, more fuel would have to go into the CWT (bringing the CG forward) and therefore more fuel would be required in the tail. The opposite appeared to be true, according to the engineer's notes.