The display uses measurements of on time of the #1 injector. This assumes a specific delivery rate, which may be off.
Based on 63K miles, I find the MFD is 1.6 MPG higher than what I calculate the traditional way, at 55 MPG fuel efficiency.
Now if you were to ask me why the MFD shows higher fuel efficiency at the beginning and end of the tank, I believe that is from the evaporative emissions recovery system. Fuel vapors in the tank are sucked into the intake manifold, thus bypassing the injectors. Now the O2 sensor initially detects too rich a mixture in this situation, so it injects less fuel (shorter ON injector time). Because of the shorter ON time, the MFD sees this as more miles per gallon, since it estimates fuel consumption based on injector flow rate, not knowing that some of the fuel is getting to the cylinders via evap recovery. This COULD account for the 1.6 MPG discrepency as well as deviation in flow rate from expected flow rate.