Here is a brief description of the fueling process and what is taking place. Click on the link to see how it works.
“Accurate temperature compensation allows dispensers to place a full fuel load on the vehicle during all weather conditions. Anytime the temperature of the gas in a cylinder is higher than 70° F, the full cylinder pressure will be greater than the commonly rated 3,600 psi service pressure at 70° F. This is allowable as long as the pressure in the cylinder does not exceed 4,500 psi during the fueling process, nor afterward, and the “settled” pressure at 70° F does not exceed 3,600 psi. While temperature compensation is a critical function for CNG vehicles that regularly operate in hot climates, it is also still important in moderate and cold weather to prevent over pressurization if the gas in the container becomes hotter than the surrounding environment was during fueling.”