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
You can put less mass of fuel in at lower SG because the tank VOLUME is fixed. Basically, if you fill the tank to the brim, the volume constrains what can go in. Thus the mass of fuel in the tank is determined from a fixed volume times the (variable) SG.
3900/0.795 and 3800/0.79 are similar numbers indicating a (near constant) volume. I suspect the masses have been rounded to the nearest 100kg.
(Incidentally, that's what galaxy flyer was alluding to; some aircraft end up being volume-limited, rather than mass limited, especially if using low SG fuels. Since mass of fuel is what propels the plane, being volume limited is quite annoying if you're trying to get a long range....)