MFD MPG calculations?

I have two questions about the MFD (or any related sensors) and the average mpg shown on the screen.

First, how accurate or inaccurate is this calculation under normal circumstances provided everything is fine with the ICE and the drive battery?

Second, where is the data coming from to generate the average mpg report, or the instantaneous mpg for that matter?

The reason I ask is that I had to replace the HV charge computer several months ago in my '09. It "popped" due to dust across the HV sensor pins. Prior to replacing the ECU, the displayed average MPG was typically ~48, sometimes a bit better. I replaced with a used ECU that was physically perfect inside and out...no evidence of connector burns, board repairs, etc. Since then, the average mpg displays around ~42. Let me state that I'm looking at these numbers from a full up to a single blinking pip.

Here's where things get interesting. I have a Carista scanner and I use Hybrid assistant. My reports always show well over 50 mpg (for normal 20-30 minute trips, variety of driving). AND when I go to fill up at the pump, I literally cannot get the amount of fuel into the tank that the MFD & fuel gauge "say" I should. When calculating my mpg based on the amount of miles I actually drove with the gallons of fuel it actually took to fill up, the numbers match the scan reports, ie 51-54 mpg.

I have had to reset the fuel gauge once (the stuck-at issue). This didn't change the low MPG display. So how is the MFD making the calculation? Obviously the HV ECU has something to do with it, as well as the fuel sending unit. Anything else it's pulling data from?

I've read a number of posts from people who have MFD's showing higher than actual mpg, but not lower. Is it possible that the replacement ECU has some odd issue?

Any information would be helpful.
Thanks.