This was discussed in another thread a couple weeks back. The tread, with all it's steel belts, doesn't seem to expand or contract much based on inflation pressure. The hub height changes and the contact patch deflects, but the tread acts somewhat like a tank track and the revolutions required to cross a given distance remains very consistent.
Except that it doesn't remain consistent like a tank track. We know this because we can increase or decrease the air pressure and then be able to recalibrate the speedometer to be accurate again.