So Chilled lead has a lower antimony content. Magnum shot with a higher antimony percentage will have a lighter mass. This is probably why the Remington Field Grade 1 Buck I buy weighs less than what is stated in Lyman's Manual.
Remington Field Grade 1 buck: 35.5 grains per pellet.
Lyman's manual states 1 Buck with 2% antimony: 40.5 grains per pellet.