I currently drive a 2015 Opel Astra. It shows an average of 5.1 liters per 100 kilometers (sorry, German, so it's metric units for me) over multiple months of d

I currently drive a 2015 Opel Astra. It shows an average of 5.1 liters per 100 kilometers (sorry, German, so it's metric units for me) over multiple months of driving in the dashboard. At the same time, I'm tracking every one of my gas refills using an app which also calculates consumption averages over arbitrary timespans. By that data, I am actually consuming about 6 liters per 100km, which is an almost 20% deviation. I thus stopped to trust the fuel consumption measurements of this particular car. Before that, I had an old and small 2005 Peugeot 206. The measurements of that car were like exact matches (at least to the first decimal digit) with my manual consumption tracking. What really baffles me is that the newer and more expensive car is the one with the huge deviation, not the older and cheaper one, as you might expect.