It's possible that traction plays only a minor role in the ct/km discrepancy. If there's more friction on concrete, the tire will heat up more, and exhibit more growth.
Pete's data shows the opposite. The difference between pre- and post- runs is double on the asphalt surface, perhaps because of the darker color absorbing more heat. The data would support this, as the major discrepancy appears in the 7/21/06 data, taken 4 hours apart, from the cool morning to the hot mid-afternoon. While the 6/17/07 data varies by less than .4 ct/km, readings taken an hour apart, both in mid-morning.
Mike, I think your thin elastic membrane model is flawed, unless you're using high-end racing tyres (spelt it properly for you). Inflation should prevent the tire from behaving like a membrane, yes?