Is the fuel level relevant when considering average fuel consumption?

Hi,
Does the combination instrument calculate the average fuel consumption displayed on the FIS based only on the instantaneous values, or does it also take into account the fuel level and its change?

Background: During the first 10 or 50 kilometers after refueling, the displayed average fuel consumption is always lower than it is towards the end of the tank. This is likely because the float in the fuel gauge probably rests against the upper wall of the tank when it's full, and only drops down after a certain amount of fuel has been consumed.

Calculate the average fuel consumption based on the fuel level, or should the fuel level be included in the calculation?
I really can't imagine that.
I believe that the actual amount injected is added up, and a fuel consumption rate is calculated based on the distance traveled. Both of these values are quite accurate, unlike changes in the fuel tank level.

I would also assume that if the described effect didn't exist. In my car, the actual fuel consumption is currently about 0.5 liters per 100 km higher than what the onboard computer (FIS) indicates. Also, the displayed range decreases faster over the last 100 km than would be expected based solely on the FIS values.