Neither did I Dave!!!
It has given me much to ponder....
If wrong darks made this much difference then maybe darks wrong by even
half a degree might affect the noise floor too.
My cooling cut in/out cycle has a delta of 2 degrees.
When I stack the darks I can see it fluctuate brighter and darker as it stacks
100s of dark subs.
This seems to even out though because the lights cycle by this 2 degree delta as well.
I'd be guessing that if I chose only the lights that were 10C (and not 11C) and calibrated using only 10C darks (instead of the 11c darks) the result
might be even more surprising.
Steve