The FQI on the airbus is one of the most sophisticated fuel indication systems that I have ever seen and had most of us on the maintenance course ( back in 1988 ) quite surprised at the number of sensors that can be inoperative and it still have a reasonable level of accuracy.
Errors are most likely to come from the fuel truck and are likely to be exaggerated by changes in the SG of the fuel, other factors that might cause errors are if the aircraft was refueled with the IRS system off line as this has an input into the FQI system.