https://youtu.be/m1YPTarNliw?t=49
Ever wondered how bullet molds were made. Here is a video for you.
Talked to a friend who is a machinist. He said that when the cutter blade is new, then it cuts to true size. As the cutter is used ( IE 250 times or more) then the cutter gets worn and actual cavity size decreases. Meaning that the bullet will way less, than the first one cut.