How many people are calculating for resized tires?
http://www.dakota-truck.net/TIRECALC/tirecalc.html
My guess is that most people are running oversized tires which should actually decrease odometer based mileage (small percentage but not insignificant... could definitely correlate to 1 mpg or more).
Based on this, I'm surprised that we don't see more numbers below 25mpg. Perhaps there is some pride in getting better mileage that stretches the truth...