I suspect that they are harder and giving them a running start before they're forced into a slightly smaller size while spinning helps keep pressures more uniform from shot to shot.
Thin jacketed cup and lead core can more readily be forced down that small twisted barrel hole.

I'll be happy to hear from somebody that actually knows and isn't just speculating like I am!

"Camping places fix themselves in your mind as if you had spent long periods of your life in them.
You will remember a curve of your wagon track in the grass of the plain like the features of a friend."
Isak Dinesen