The entire business of precision can be rather non-intuitive ... :-)

My car is reported as 51.5 in the GH DB, but is 51.355 by my arithmetic. I take the lifetime odometer reading at last fill-up / the sum of fill-up amounts (recorded to 3 digits after the decimal on each tank. My mileage has a precision error of 1/miles traveled; the gas consumption 1/1000. Since I have traveled over 1000 miles, the gas consumption becomes the major source of error -- ergo, 1/1000.

Off-hand I would guess the error in the GH DB is due to summing the rounded tanks, which introduces cumulative error. I agree with you that it is not much greater than 0.1 mpg lifetime, because the rounding errors on each tank tend to cancel themselves out over time.