When I first operated the B737-300 for Orion Airways many moons ago I also contemplated this anomoly. My research at the time was that the capacitance type gauges were accurate but I decided to drip the wing tanks having filled them to capacity. (I managed to borrow a step ladder from the friendly refueller at CFU!).
Althought the gauges showed 9,300 kgs the dripsticks showed 9,100 kgs! I managed to persuade the company to lower the full wings fuel figure to 9,100 kgs since the early B737s were very obstacle limited on take off and the 200 kg could be the difference between a tech stop or not!
I suppose the academic way to refuel is to drip the tanks prior to commencing the refuel, obtain the S.G. and calculate the fuel volume required (eg litres), pump the fuel in and then, to be belt and braces, drip the tanks afterwards! A gross error check would then be made by referring to the fuel gauges. In the "olden days" on the B707 we had a flight engineer who did precisely that!
Oh, by the way, never had a problem getting the SG from the refueller in circa 25 years of operating!