Varget is extremely temperature stable, your velocities shouldnt vary more than 10fps between those two temps, assuming everything else being the same (what I have heard is about .2-.25fps per degree F), and #2, your hot weather velocities should be HIGHER than the cold weather velocities, again, assuming everything else is the same.....
I think its safe to say we can rule out the powder being the cause. (which rules out the ambient temp)
You simply cannot directly compare results achived using different brands of brass
My guess is the fact that you used different cases is the factor you are looking for.