Do you use hypermiling? I am not talking about amateur "change gear early" hypermiling. I am talking about a full DWB, P&G, and FAS routine with a proper setup, i.e. 0w-30 synthetic, 75w-80 gear oil, and 44-60 psi tire pressures. Your car should be in top notch condition as well. These results are impossible otherwise.
Please check whether your car is not burning petrol and CNG at the same time. Also, tell us whether your carburetor is stock or not.
If not, then either your odometer is out of calibration, or you are reading it wrong. If possible, check the results again with a GPS maps app.
BTW, 800 km is more than half the distance from Karachi to Islamabad. No car that follows the laws of physics can do that journey in less than 2 tanks of CNG.