In generation II Prii (model years 2004-2009) there is a gas bladder which expands and contracts with the gas volume found inside the gas tank. It does this so that it can keep the overall volume of the tank lower to prevent the presence of gas vapors. By limiting fuel evaporation, you save money on gas, since those vapors which you paid for in the form of liquid gas can leave the tank during refueling, also impacting the environment.
Sometimes the bladder does not fully expand during fuel up, and with the overall volume of the tank variable (read: smaller), you will pump fewer gallons until it's "full", and the math will give you a higher mpg than reality. It all works out over multiple tanks, but can and will give inaccurate calculations from time to time on individual tanks.
Reportedly you can help the cause by filling up slowly.
If you own one of these and are wondering if you should fill up, the answer is yes. Lesser men have run out of gas. Your pre-fillup mileage calculations mean nothing with the Prius "guess gauge".