John those are really good points and one reason a lot of this temp sensitivity business will drive us crazy chasing it. And then when you change lots of powders and see the variations it's sometimes hard to evaluate what you're looking at and what's causing the variations.
Frieds and I are now seeing lot to lot changes in H1000 in the 7mm Mashburn; I noticed it about a year ago as my old lot dwindled and I tried some new stuff. The old lot gave 100 fps more velocity. They are noticing the same thing so I don't think it's temperature since I've shot the new stuff side by side against my older ammo.
It's hard to be a backyard lab on this stuff.
I remember a ways back when I noticed this temp stability business, I recall IMR4831 showed more velocity loss in the 7 Rem Mag with 140 gr bullets than it did in the 270 with 130's. This was pretty consistent over maybe a couple of winters, since I did a lot of winter chrongraphing back then. I often wondered if load density had an effect on it,
I also recall that 7828 did pretty well in the 7 Rem Mag with 160's going from winter back to summer temps, It seemed more consistent than either IMR4831 or IMR4350, but like you say it might have been the load and cartridge.
What you have observed is the reason I will replace older powders with the newer temp resistant powders but not until I burn up the old stuff...
