From what I understand the 737 tank sensors do not compensate for different S.G. of fuel being uploaded, therefore it is common to see the wings showing less than full when they are actually full (valves automatically shut whilst fuelling when the tank is full).
We calculate an uplift litreage using the S.G. and round it up to the next hundred. That becomes a minimum uplift from the bowser, no matter what the guages say in Kgs.
PP