I have two theories on how they did it.
Theory one: They inflated the balls right before with very hot air. Over time, the air cooled and deflated. But the refs couldn't feel it when they tested, since footballs are fairly insulated.
Theory two: They inflated the balls with 10 lbs of air, and another 3 pounds of some other gas that permeates rubber more easily, so that by game time the other gas had leaked out.