First of all, it all depends on the type of aircraft they're refueling. (300kg of fuel on a large aircraft is not as important as on a smaller one). In my case A340/330.
In my company, we write the amount of fuel needed in Lts in the technical log book, that has to be signed by the commander (having in mind the fuel density provided by the refueling company) and that amount added to the fuel over destination has to be consistent with the one required on your OFP.
Further, when you add, lets say, 40 Tons (at a mean temp of 24ºC) to 15Tons of Fuel remaining in the tanks, that is at a temperature of -36ºC, (after 8 or 9 hours of flight), you will get a lower weight than expected, but the required amount in liters will be as predicted. Since most aircrafts give you a "weight figure", that has a fixed .800 "cadensicon" fuel weight determination, you may then find a different figure on your FQI. That summed to, at least, a 1% (depending on aircraft) FQI margin will probably give that amount of misreading.
Anyway I must tell you that, using the fuel density liter/Kgs determination procedure we have found that some of our refueling companies in Africa were cheating us...
Regards,
VF