Interesting.
Keep in mind, you can have other factors influence your results. The most obvious is the angle of the sun, relative to your chronograph. Other factors like differences in barometric pressure from one day to the next may have a small influence as well.
I did a mini version of your test several years ago, using H110 in .300 Whisper. To minimize these external influences, I put a metal mag of ammo in the oven and warmed it at 150 deg. The mag then went into an insulated lunchbox on the way to the range. A similar mag of ammo was left out in the cold.
I would have to dig for my notes, but I want to say the difference in average velocities was less than I had been led to believe with this powder.