Most people then they gage their MPG will let the auto shut off on the gas pump decide when they are full.  I have noticed up to 1/2 gallon difference from pump to pump in the auto shut off.  To somewhat remedy this (even on long runs) I top off the tank until I the pump will not pump any more...every time.  I also think the ambient air temp and the fuel in the tank temp will affect the pump auto shut off.  If your run is not at least 120 miles and if you do not really top off the tank (before and after) the results are very suspect.  Additionally, some gas pump parking areas are on an incline and so then will your tank be.  If you really want to get to the bottom of this you need to consider all the variables that can skew the data.  A small sampling is a huge problem.