Why so much diff in seating pressure needed?

[AlNyhus]

Don't turn necks.
Anneal every firing.
Seated past neck shoulder junction

Have you checked for a 'donut' at the bottom of the neck?

Also, cleaning the inside of the necks to the brass can change seating pressure...even if you add graphite as a lubricant. You might try not cleaning the inner necks and leave the carbon buildup. Just one quick twist with a nylon brush is all you need. Take 5 cases and shoot them a few times and see if it helps.