100 fps is a pretty big jump in velocity , I know that the warmer air is less dense but I seriously doubt that even a 60-70 deg differance will make a 100 fps differance in velocity. I would wager to say that your big jump in speed is due to the powder being temp sensitive more that the air density .
BUT , not sure if you had shot this same load in cooler weather but alot of times some barrels will shoot a good bit faster with a given load than what the books say. I have a 25" 1-11 twist Obermeyer on a custom 300 win mag thats shoots almost 100 fps faster than any of the books loads with a 26" barrel. , some barrles are just fast.
I know a guy that still hase a Remington 700 , fully blue printed and had a 26" Pac-Nor barrel installed , chambered for 22-250. H built the gun just to shoot 50 gr bullets as fast as possible for ground squirrels. he worked up a very hot load in January (30 deg) with H-380 that was flatteniong and loosening primer pockets with one firing. The gun sat dormant till his trip to California (July 90+ deg) where he took it out to check zero and fired it one time and it galled the bolt closed. He has since made the gun into a decoration for his reloading room
