I have a new theory. Toady I drove again and noticed, that the average consumption value was changing very quickly. So my new theory is, that whatr happened two days ago was the following:
The average consumption display reset itself two days ago for unknown reasons in the middle of the drive. Because I was driving steeply uphill in second gear at this time, it then suddenly went up to 10 l. Now that I have driven some km in the city (but always stop n' go) it went down to 8 l again. I guess during the next week it will go back to about 5.5.
Still it's strange that the average consumption display reset itself without any reason in the middle of my cruise. (It's configured to "manual reset")