Depending on how long the car's been parked and temp differences, tire pressures can drop easily. A 5 degree temp change down can cause a 1-3 psi drop. That's why tire cos always preach check tire pressures monthly. When you start off cold in the morning and drive 5 miles tire pressures go up 2+psi - always check tire pressures cold.
Re: Gas station air pumps - they're pretty questionable and their "customer use" pressure guages. Alot of those outside air pumps get moisture in the brass coupler and freeze and don't always open when you attatch to your valve stem.