something wrong with your app calculations there Birdy. I get your average on the left to 19.67? As for how the calculation is done - i'm guessing the reference point from a full or empty tank verus how many litres have been put in and used versus the correct number of miles is the only way you can do that.
Mine does about that much. When the electric fuel pump starts banging I look for a petrol station as it means I only have about 200 yards of fuel left. I then put about that much in and it works out that i'm getting around 20mpg. No science, fact or mathematics used in the above calculation
1641 engine in my early with standard solex carb (both been rebuilt by me recently) gets me 24 mpg which ever way I drive it 8)