Air is a very poor conductor of heat. That's why you can be standing outside in 15 degree temps, 10 feet from a campfire and feel the heat (infared radiation) yet the air between you and the fire is much cooler than your face. Since the IR is reflected BACK to the plywood which transmits the IR back out to the shingles and surrounding atmosphere the attic is much cooler. It's physics plain and simple. Just because you might not understand it doesn't mean it doesn't exist.