Maybe it was the expansion/contraction properties of the Rotor that were subject to change from the cold weather casuing it to shring from 109mm to 108.9mm
or if they were digital calipers the battery volts were possibly pulled down in the cold casuing the reference voltage to drift and an error to occur
yep I know i need bigger wotsits to run nearer the front, then again I hear dodgy eyesight helps as the gap may look bigger than it actually is even if its only 0.1 of a millimeter