You cant really believe what the DIS says if you are running anything other than a standerd car.
It does its calculations on the assumption that everthing is factory.
Chipped or "tuning boxed" cars are injecting more diesel per stroke but the ECU is blind to this.
If you want accurate fuel economy calculations you have to go tank to tank and fill up at the same garage, at the same pump, with your car in the same direction to the click.
You have to reset your trip miles to zero then when filling up again use this number and the amount of litres you brought
mpg = miles /(litres of fuel x 4.545)
This will give you a far better indication of where your car is at!
And if economy is your thing use vagcom to adapt (lower) the amount of fuel per stroke.