I suspect that most of the variability you're seeing is in the bullets.
All seating stems engage the bullet below the tip so unless the bullets are extremely consistent (from a single die) they won't measure the same at the tip.
A bullet comparator may give you the info you're looking for.
link here



"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