Tire Pressure is always a moving target. Sometimes they check it when you're getting your oil changed, sometimes they don't. Half the time, the air filler gizmo at the gas station is broken, or takes quarters and I'm fresh out of change. I'll fill them next time through. Days turn into weeks. Weeks into months. You take a train track too hard here, a pot hole there. Cold temps, hot temps. Sometimes, your tire pressure is 10 pounds below.