Don't forget about the confounding effects of local and seasonal differences in fuel formulations - that will introduce a significant variability in what mileage different drivers will get. Not to mention changes in formulations gas over time (for example, they used to put MTBE in our gas here in CT, then they switched to ethanol. I don't know which would give me better fuel economy, but I'm SURE the two additives give me different mileage).

Bottom line - don't get hung up on differences between your observed mileage and the mileage someone else says they get, because there are too many variables you can't account for to make it a useful comparison.