I figure these are for muzzle loaders and muzzle loader bullets should be made of pure lead. You may have simply squeezed the handles a little tighter on some of them. They temperature of the mold or melt changed for some reason. Adding metal ,even the sprues can do this for a few seconds. A cold breeze can really cool the pot. A change in the amount of drop to the mold can make a difference. You seem to be within +/- 2% so it should not matter a whole lot. You will get better as you play with casting. Try a few of each weight for group ,then try a mix of weights to compare them. If the narrow difference makes a difference in groups I'd be surprised.!