HOW much more powder did you put in? Looking at Hodgdon's data site a max load of H322 for this bullet is 23.5 gr.
Did you keep the overall length the same? Did the shells sit out in the cold longer for the second test? Are your chrony screens set the very same every time - some models the operator has to set up screens and it can be different each time by a little bit (like my old Ohler). Did you clean your barrel between tests? Was the bore oiled? How cold were the batteries in the chrony on each day? Was the first screen exactly the same distance from the muzzle each day? Were you using the same lot of brass from same maker?
I have done chrongraphing down to 0 or even colder - not any more as its no fun. I had my chrono set up in a box that is insulated from the cold, &/or put a hand warmer in the box to try to keep it from getting too cold. I try to keep the shells inside my coat until placed in the gun to shoot. But even so there can be some variation depending on if the gun has gotten real cold or is warm/hot from shooting, fouled from shooting or gooped up from cold stiff oil in the bore.
Lots of places for variability to come into the picture.
And that is all ASSUMING you really only raised your load by 1 grain not 10. It appears that currently made H322 is one of the extreme powders and should be relatively lightly affected by cold temps. Is your powder new or could it be old pre extreme stock? I don't know much about this powder so can't speak to its performance. I have seen considerable differences in performance over different temps with some older 4831 and 4350, possibly 4895 but I don't think it was 100 fs or a decline with increased powder.
Lots of questions that might help you home in on some sources of variability.
Hope this helps a little but I sure can't answer you more precisely.
Maybe more experienced folks will chime in.
