Have heard two theories, both from various industry professionals, one about bore dimensions in real cold. The other is that the instant a warm bolt-face touches the very thin cup of a primer, it changes the way the primer ignites slightly, since the priming compound contacts the cup.
I tend to believe "field" results, shot with everything at the same temperature as they would be when actually hunting, over simulated lab results--which is why I started testing handloads in actual environmental conditions in the early 1990's. One thing that tends to confirm the field results is that POI of the loads that don't chronograph the same can vary. Not always, but that often depends on the rifle, not the velocity. Many rifles do not change POI at 100 yards even when muzzle velocity varies 100-150 fps, but others do. As is demonstrated when working up handloads with various powder charges.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
