First, it is likely a percentage off between the two, and in a car with better fuel economy the raw MPG number difference will be larger. But that seems too large to explain this completely.
It could be that the software was written for gasoline. The program is tracking injector pulse width on one of the cylinders to track the amount of fuel being used. The difference in viscosity between diesel and gasoline means a different amount of fuel gets injected. If the computer thinks it is injecting a the wrong fuel, there can be issues.
I also see your fill ups are a bit erratic in fuel economy. Try recording the dash display results in the notes, and we might be able to see a trend on when it is off the most.
As to VW shenaigans, isn't the 1.6 only of the cheater engines there? The dash display being off could be an artifact of the cheat code. Not necessarily delibrate on VW's part.
__________________
|