VW Golf 7 110TDI fuel consumption problem

[gti mk4]

Hi All,
My car has done about almost 6000kms since I bought it brand new two months ago. I have been observing the fuel consumption for a long time.
However, I have to fuel it up for half tank after driving 300 kms. Hence, the estimated data will be 750 kms for a tank diesel. My old mk4 GTI does about 800kms per tank. On the book, it says fuel consumption combined is about 4.9L/100km. It suppose to give me 1000kms/tank. While I have a look at my computer and it says 6.2L/100km for long term and 6.4L/100km for refuel and 6.4L/100km for since start. I have refreshed the data many times and kept looking at the data for the recent two months and never lower than 5.6L/100km on average.
The weird thing is when I look at the instant fuel consumption and it says about 4L/100km. I am wondering why ? For a 2 litre diesel car, why it takes so much fuel from the engine. it suppose to get like 1000kms/tank because my mate's MK5 Golf 1.9TDI does that 1000kms/tank. I have also tried BP, Shell, Caltex and they are all giving back the same result (less than 800kms/tank).
Is anyone having the same problem? Who knows what's going on? Thanks