They did spot a discrepancy...
Although the aircraft's cockpit and refuelling-panel indicators showed total fuel of 86t, the refuelling had taken longer than expected, and the ground personnel found that the dispenser had apparently delivered 121.5t to the aircraft.
The ground team was uncertain about the discrepancy, initially believing that the fuel-flow counter might not have been reset before the fuel was dispensed.
This prompted them to dip the centre fuel tanks and somehow managed to come to the same 86t figure the aircraft came to. Their logical but false conclusion must have been that the discrepancy lay with the fuel bowser and not the aircraft.