It'll be sensitive to how level the parking spot is. The dipstick is all the way on one side of the pan. Can you park always in the same spot?

It'll be sensitive to how long since you shut off the engine. Not only the temperature but how much time the oil has to drain down from the head & everywhere else. Try to be as consistant as possible when you're recording oil levels. Maybe record oil level in the morning instead of at all the different gas stations.