I think Sebach is on to something. After topping off a tire a few times, it seems like they hold pressure longer. Perhaps the early deflation is the oxygen leaving, so after a few top offs, you've got a much higher nitrogen content?

Sorry mentor58, I don't think it works that way. If you take a tube of air into a room with higher pressure, it will crush the tube. You would need a rigid tube in order for a higher external pressure to "inflate" the tube.