Keep in mind it is IMPOSSIBLE to fill your gas tank to the same level two times in a row. If you put in more gas than expected this time, you put in less than you thought last time.

Most ( if not all ) gas tanks have air in them. And depending on how much air escapes, or does not escape while fueling, you get vastly different amounts. In the case of the Ford Escape Hybrid, there is 3 gallons of air space in the tank. The tank is labeled as "15 gallons" but most people can never dispense that much. If you park on a slope, with the fill tube on the high side, you can ( and I have ) dispensed 17.5 gallons.

-John

Also keep in mind, nearly all MPG multi-functions displays on cars read HIGHER than real. This may be to make the driver ( who never checks at the pump... aka most people ) feel better about their car.