I believe the answer is the inside taper of the hull. Federal, Fiocchi, etc. are said to be "straight walled". Winchester AA, Remington STS etc. are "tapered wall". So, wads are made with slightly different diameter at the base to make a good gas seal for the two different types. Not sure the basewad makes a difference to the wad, they don't touch for any load I've ever done. I'm so sure I think I'll head down to the loading bench to mike a couple different wads!

"Every one must believe in something, I believe I'll go hunting today."