Dude its a full time 4WD version, company has designed it in such a way that the drive force distributes equally in all the wheels, as u hav removed the rear drive shaft now front differential + axles has to put extra effort in order to drag the car.
u r calculating it in a wrong way, just go to Petrol Pump & ask the guy to fill the tank (full) then reset ur ODO meter Trip A/B, drive the car for a week, then again refill the tank (full), this time note the Liters (Dispenser)
Petrol: 28.5 Liters
Trip A/B : 444.6km
Mileage : 444.6 / 28.5 = 15.6 km / L