S.B.
I understand you want to know where this information comes from.It should have read "new lot# of powder"the free manuals that powder makers give away,I have read it in there.I have also read it in numeruos shooting rags.Right now I could not give you a specific place to go to and say there it is.If you have old manuals (reloading)check powders in new upto date manuals against the old one and you will notice a difference in starting and max loads.some lots are hotter then others and some are not as hot as others,but they keep them in a certain spec area that they are intended for.But,and,is why the warning. new lot # start over, start at min.and work up..
I find that some of the oooolder manuals (reloading)the give away ones.have a wealth of trivia info that the newer ones do not.I have some dated from the 1930s..