I started trying to think of reasons your times might be systematically different (other thin bike calibration, as you noted) and every one I could come up with

I started trying to think of reasons your times might be systematically different (other thin bike calibration, as you noted) and every one I could come up with -- altitude, CO2 level, noise level, how you get there -- could affect anyone at any time. If you're not racing next to me we're not competing to any degree of useful accuracy.