OEBS aside, we've had a history fuel spills which we simply can't explain. We've considered the entire gamut of reasons. Airbus have loked at everything and were no help. We looked at density/temp issues - sensor faults, ramp gradient in both axes - , all we're left with at this point is that the fuelers are fueling in manual mode instead of in automatic. Proving that after the fact though is near impossible. In the end, we switch pumps off for refueling. Spill incidents have practically disappeared so we're content with the status quo.