Fuel consumption: on-board computer is next to it

Hello together, I made the statement in my XC60 that the specified fuel consumption usually deviates strongly from the calculated consumption. Usually it indicates less, that make the from Volvo certain extra real consumption I consume about 0.5 to 0.8 liters more, than the BC indicates. I find that the deviation is too large here. Everything in the range of +/- 0.2 liters I would still accept, but I don’t need a treasure ice. I once read that the display should be more reliable on long journeys. When I chased a tank filling through the engine recently on one day, I still had 0.8 liters of deviation – the BC showed 9.4 liters, I calculated (and Spritmonitor.de) 10.17 liters. How are your experiences?