The air in the tire has likely experienced a change in temperature. When the temperature of a gas changes, its pressure changes as well, according to the ideal gas law (PV = nRT), where P is pressure, V is volume, n is the number of gas molecules, R is a constant, and T is temperature. As the tire cooled down overnight, the temperature of the air inside the tire decreased, causing the pressure to drop. When the tire was driven to the gas station, the friction of the tire against the road warmed up the air inside the tire, increasing the temperature and causing the pressure to return to its normal value. It's important to note that if a tire repeatedly loses and gains pressure, it may indicate a slow leak and should be checked by a professional.