I doubt the car references the fuel level for average consumption - only distance to empty would. Going up and down hills moves the float in the tank.
Surely with EFI it "knows" how much fuel gets squirted in for each combustion cycle (4 bar pressure, pulsewidth, injector flow characteristics), and the wheel speed sensor combined with the programmed tyre diameter allows the distance calculation to take place. Compare the two and you get fuel per distance.
Interestingly I found before custom tuning the car reported a higher than actual fuel use. After custom tune it reported a lower than actual use. About 0.5 to 1L/100 either way.
As said the only way worth checking is to fill it right up, drive and fill it right up again and record odometer before you head off. I used to write it on the fuel docket.
Haven't bothered for years though. I just reset Trip A each fill up and take a bit of a look at the fuel needle as I'm driving around, say at 3/4, 1/2 and 1/4 (if I let it get that low) then guesstimate.
Trip B records distance since last oil change