21.25 km/litre for BMW x2 18i

I did to and fro from Cologne to Kerkrade (85 kms both ways). After I reached and checked I burnt 4 litres of gas. The average comes to 21.25 km/litre. My friend said it cant be real. Today I returned and checked, again it’s 4 litres consumed. Is this realistic or something wrong with the app ?

What average have you got on highway driving?