displayed versus computed mpg 2% high

After tracking my fuel usage very carefully for the first two and a half years that I owned my 07 Prius II (recording gallons pumped, miles driven and displayed mpg for that tank and then resetting the car's trip odometer and mpg calculator, at every fill-up). Over that time period the average of the displayed MPG was 49.1 mpg, and of the average of the hand-calculated MPG was 48.0 mpg; this is a 2.2% (i.e. about 1 mpg) overestimation of displayed fuel efficiency. Such a systematic error must be due to some combination of car computer overestimation of miles driven ( e.g. due tire diameter smaller than assumed--all aftermarket tires I've bought recently are about 2% undersized), and/or underestimation of the amount of fuel inject per inject. Or, since gas station pump meters cannot be perfectly accurate, maybe they tend to be calibrated to overestimate the amount of gas pumped (its probably safe to assume that a station owner would tend to err in his favor, if he was going to err). Whatever the error source, since it is systematic, Toyota should be able to easily correct for it, and in fact it would be nice if the owner could enter a wheel diameter calibration factor, such as is used for a bicycle speedometer/odometer.