That came up a while back on one of the threads. Mostly everyone seemed to be converging to errors in the reporting of the mileage by the car, though I think if thats really the case some people would be seeing the reverse (lower computer calculated than hand-calculated). I really don't have an answer as to why that is so, but I have observed that its consistently overreporting by about the same amount and also a lot of other cars (different models, not necessarily hybrids only) also tend to overreport MPGs.

Perhaps more gasoline slips through to the engine than the meter detects?