a false S.G. of delivered fuel is a good way to fiddle refueling to get the figures correct.
used to refuel B747 in KUL to LHR and uplift was over 200,000 litres, so the forth sigificant figure of the S.G. being inaccurate would give a 100kgs discrepancy.
and it is very hard to get an accurate reading on the hydrometer on the forth sig figure, as the hydrometer sticks to the side of the glass jar and the meniscus of fuel on the scale trying to be read.
if the S.G. of delivered fuel is supplied by the refueling company and not measured by an airline employee, then will get fiddled.
An old tale when company aircraft were refuelled in New York and always got a descrepancy, was that refueling was started before the pilots arrived at the aircraft, so would not know if the numbers had been zeroed before delivery.
At certain airport the local mechanics were over zealous with draining the water out of tank and could fill a 45 gallon drum with waste fuel.
If a discrepancy always happens at the same airport to all aircraft then suspect a scam.