The temperature setting does not change the temperature of the air coming out of the vents, it's the temperature that will trigger the heating or cooling system to stop because the room has reached the ideal temperature. So if it's 80 degrees inside your house, and you have the number set at 72 and 'cool', what you're saying to the system is that you want it to produce and spread cool air into the house until it's 72 degrees inside. The cool air coming from the vents isn't 72 degrees, it's cooler. When it reaches 72 degrees inside, the cold air can stop being pumped in until it gets hot again. But when it's 80 inside, you have the number set to 72, but you have 'auto' on instead, what that tells the system is that you want air spread around your house until the air inside is 72 degrees, BUT because you've got it on auto instead of cool, the system doesn't cool the air down before it spreads it around the house. So it's warm room-temperature-ish air that comes out, and it will likely stay on for a long time because it doesn't turn off until the room is 72 degrees, which isn't going to happen.