You mean the petrol average which is shown on the gauge cluster? That shows instant mileage and calculates average mileage on the end on the trip... Here again an average is taken for the whole trip.
In cng your trip duration is the distance traveled in whole cng tank consumed.
The variation in every trip can be due to many reasons like cng quality, pressure, car tuning, tire pressure, driving style, terrain, ac, occupants and loading, ambient temperature and pressure, etc etc