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.