Yes, correctly said by asim0072, when gas is filled touch your cylinder, its quite hot. and in morning its at normal temperature. so lower is the temp, lower becomes pressure.
therefore its said to fill gas at early morning or at evening time, so you get "thandi" gas. or the best way is to fill the gas at nite, park the car and let the gas be at lower temp. coz lower temp gas gives better mileage.