I've just started to notice, that after I have filled up, pressed RESET on the Fuel Economy bar-graph on the nav screen, that after a bit of driving there's a discrepancy between what the Bar-Graph show and the "AVG MPG/TANK" on the Mult-Info Display in the center of the speedometer. Does anyone know why this might be?

AVG MPG on multi display sometimes takes a little bit longer to auto reset (after I already started driving after fill up) and that caused discrepancy on my car. I would speculate it waits for tank to be repressurized and gas level to settle down to make sure the tank was actually filled up. Anyhow the discrepancy looks huge for first few minutes of driving and then disappears completely after some more driving. Outside of this anomaly, which is not always present anyhow, both displays are always exactly the same if reset properly. Don't know if this is your case also or something else altogether.

I filled up, reset both displays, then a mile or so later reset the NAV display again. Mine have been different by .5 mpg since then. At the next stop, I'll reset both again, then leave it alone.

"When you get to the fork in the road, take it." --Yogi Berra

Why reset both at the same time. I use the fact that I have a resetable FE gauge to allow me to monitor different driving techniques and how each trip is doing vs the tank average. Some times I let it go multiple tanks to see what the long term mileage is. I just don't see the need in having two different gauges give me that same information.