I think it all depends on the way the car calculates. My ford average fuel consumption thing goes down when the car is stationary, the Alfa doesn't. It doesn't appear to count time at idle.
Say I reset the average at the petrol station, then I pull away to the junction, the display in my CMax will show about 20mpg because I've moved about 40yards in 1st or second gear. Then at the junction, if I get stuck there, it goes down quite fast. The car is using fuel, but I am not moving. Can get down to 10 ish.
My Alfa just stops changing when I stop.
Also, my Ford will show an instant mpg of 99.9mpg (3digit display)?when off throttle say coming towards slower traffic on a motorway, or losing speed for a turn off. The Alfa doesn't change much, (slowly easing up to 40mpg or so, however, all modern management systems cut the fuel in these situations so you use none the figure should be the max display able as you are doing miles with out using any gallons.