I don't trust the "on dash" computer fuel consumption figures. Try the old fashioned manual calculation next fill-up, I'll bet the numbers are higher.
The overall 10.5 L/100 km
was calculated using receipts and odometer, not the in-dash trip computer. Only the 8.5 for highway trip was referencing the in-dash figure. In this case, the in-dash seems to be giving a fairly accurate result.
Others I've found tend to take longer distances (and several fill-ups) beofre the manual calculations agree. after several long-term tests, in which I've always maintained receipts to calculate fuel consumption, every car's manual calculation has landed within 0.1 or 0.2 L/100 km of the trip computer calculation.
Just because you have trust issues doesn't mean they are not accurate. Then again, I don't trust them entirely either, which is why i always track long-term cars manually as well... we're working on a story about that, results should be in soon...