Why can the vehicle range change from a filling to another?

Why can the vehicle range change from a filling to another?

The difference between two different fillings depends on both driving style and quantity of fuel supplied. The engine efficiency and the atmospheric conditions (air pressure and humidity) influence the engine performances too.

Being CNG not liquid, the fuel quantity supplied is not only a matter of tank volume (capacity of cylinders in litres) but also of pressure and temperature achieved at the end of filling; so, fuel quantity supplied can slightly change every time (the same happens in case of liquid fuels, but in a less sensible way).

The quantity supplied also depends on gas features (see next question).