I have a TCH, not a Prius, but I can offer some light on this...

Some gas stations actually pump much less fuel than the pump claims. The gas station near my house was short-pumping by more than 15 percent last winter. It was closed in February after a state inspector tested the pumps. The gas pumps were sealed and locked by the state so that they could be used at all. Those pumps have since been removed, and they are in the process of installing brand new pumps.

At some gas stations, my hand calculations agree with the mpg figure from the nav system to within just a few percent. Because of the fuel cell in these cars, the fill level actually varies a little each time, so some variation is to be expected. If one particular pump disagrees with the car for two or three tanks in a row, in the same direction, that is cause for concern.

Some gas station operators adjust their pumps to make more money. With old mechanical pumps, the calibration adjustment was a screw inside the unit, and the state could put a seal over that screw to prevent changes. With the new electronic pumps, the calibration is in software. With those pumps, it is possible for an operator to adjust the calibration without being detected, until a state inspector shows up unannounced with a calibrated 5-gallon gas can.

In Florida, the state is supposed to test every pump every year. They don't, because they just don't have enough people, or maybe because the people they do have work very slowly. In any case, they actually test pumps after complaints are filed by consumers.

If you have reason to believe that a station is short-pumping, you can complain to your state department of weights and measures. It might take some looking to find the right department contact information in your state. Every state has someone who is in charge of testing gas pumps. In some states, the contact information for that department is on a sticker on every gas pump.