Dunerjeff beat my post, but I believe he is correct that you are seeing an artifact of case capacity change with bullet seating depth difference. In a short straight wall case, running a bullet design that seats a tenth of an inch deeper than some other of the same weight will reduce the powder space by a much bigger percentage than the same degree of difference in bullet seating depth will do for a rifle case with much higher powder capacity. That large change in space percentage can have a significant effect on peak chamber pressure.
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