Come on, we ain't fillin' up cars here.
With a proper calibration at zero fuel the upload for the first flight afterwards and the doomed flight would have been CALCULATED and THE AMOUNT IN KGS ordered and filled up from the fuel truck. I don't know about the fuel panel on the ATR, but the AMOUNT filled in from the truck would have been checked against the fuel panel indication on proper operation.
So either no calibration, a loss of fuel records, mishandling on filling up or/and a car like fill up?