Our idle pressures are about 15 lbs as well, at 900rpm for mine and 1200 on my buddies. I believe it is due to the lifter to bore clearance on our engines. The last engine i built was the same rotating assy and besring clearances, but with Gaterman hyd roller lifters that measured amlost .0008” larger than the comps and it had higher idle pressure. So that confirmed it for me.