Consumption indicator lies until the bars bend

Found an empty motorway today, really super strong and needed 2 hours from Bern (CH) to Frankfurt/Main. 55 minutes.
Run my 3.7 AEW with Quattro and Tiptronic.
According to the on-board computer, what a word, the consumption was 11.9-12 liters.
In fact, when refueling charged at 18.29 liters, which is also OK given the high speed and the driven average.
Why does the GRA display lie and what can I do about it???
In the city traffic it shows really, so always between 14-15 liters.
Maybe somebody's got an idea.

Florian