I never put any faith in those on-board fuel economy readouts. I always view them as "interesting to know" tidbits. Probably only good for "miles to empty" information.
If you are at all interested in actual fuel mileage, the hand calculation using tank fill up gallons and trip miles will be the most accurate.
I agree. the only read-out I use is the real-time fuel economy. I literally never drive without it. It's tremendously helpful for my persistent hypermiling techniques.