When my 2005 was very new ( less than 5000 miles ) the Nav screen fuel economy was ALWAYS about 1 or 2 MPG HIGHER than actual. Since this was computed over many, many tanks, it was not due to my methods of refill at the pump.
Lately, it has been much closer, like within 0.5 MPG of what I pump.

I do agree that it is IMPOSSIBLE to fill up exactly the same way twice in a row, as there is at LEAST 3 gallons of air space in the tank.

Some people have filled up with 17 gallons when parked on an incline, and I have filled up with even more than that when I used a gas can to slowly "dribble" gas in, letting air burp out.

14 gallons is a typical fill up, with 15 or 16 gallons possible. More than that is a bit tricky.

MTE 0 = you have about 1.0 to 1.5 gallons of usable gas left.
If you are in the city, don't feel afraid to take it to MTE 0 all the time.

So while I think the accuracy of the screen is +/- 2 MPG, your accuracy of pumping at the gas station is worse than that.

Most old-timers on here ( had the car a year plus ) trust the screen if you reset it every fill up. The screen only holds about 2000 minutes of data, then data is tossed out... so you really can't keep a "running total" so you might as well reset it every fill-up.