Don't know what or where you're operating, but I had a similar case not long ago at a large West Coast airport. Fuel ticket indicated 21,000 gallons uplifted, and FQI indicated 208,000 on board. Usual delivery is around 24,000, all else being equal.
Modern large Boeing aircraft have density measuring system in all tanks, so we used that of the CWT to measure the density of uplifted fuel and to figure out that the fueler had just supplied 3480 gallons of free fuel. Stick the tanks to verify actual levels on board, and done.
This has happened 3 times to us in the last 10 years. One time the hydrant truck meter "forgot" 11,000 gallons on a -400 delivery. Of course, each time the offending unit has been pulled off the line, and a flow check done, which always comes up within limits (~ 10 gallons in 25,000) so it's a mystery.
Just for reference, there can be a range in density. I've seen values from 6.68 to 6.90, depending on various factors.