Is everyone sure they are filling their tanks exactly the same? Fuel is very susceptable to heat, is your gas in the tank the exact same temperature as you initially filled? If the gas in your tank is colder than the last time you filled it will take up less space allowing you to fill the take more, resulting is a lower looking mpg. Mine is always really close. I also suppose there is a % that Ford might be off reguarding quality control with fuel metering.
I reset the trip odometer for several tanks early on and compared pump gallons to trip gallons. It was always within +/- 0.5 gallons for a tank fill. Most likely the difference in top-off is the bulk of the variance so after about 8 fill ups I stopped keeping track of it and I trust the instrumentation.
Remember the meter on a fuel pump has a range of error as well.
"In the United States, the National Institute of Standards and Technology (NIST) specifies the accuracy of the measurements in Handbook 44. Table 3.30 specifies the accuracy at 0.3% meaning that a 10-US-gallon (37.9 L; 8.3 imp gal) purchase could vary between 9.97 US gal (37.7 L; 8.3 imp gal) and 10.03 US gal (38.0 L; 8.4 imp gal) as to the actual amounts at the delivery temperature of the gasoline."