Happens all the time. The manufacturer specified capacity of a tank is different than its "real-world" capacity. There are all kinds of measurements around fuel that are like this, including octane rating. It'll drive you crazy! The "usable" portion of the tank, that is, the "real-world" fuel capacity, can be higher or lower than the rated capacity. Does the pump click off at the exact expected time the manufacturer designed (of the car and the pump), is there emissions equipment in the tank that was not included in the measurement, did the driver top off (don't), is the fuel cool or warm (this actually has very very little to do with fuel amount, about 0.9% at very most, but still, a few % here and there can matter. The reference temp is 60F) Pumps are normally checked by the state every year. It's very probable there is nothing wrong with the pump.