Accumulation of tolerances. Meaning no matter how hard you try everything is not ALWAYS going to be the same.
Just a few examples: Fit of modified case to chamber, tension applied inserting modified case, straight alignment of gauge into chamber, tension used to tap bullet into lands, concentricty of bullet at the point where contacting lands, etc.
I have used the Stoney Point (now Hornady) for years and IMO it is still the best way to determine seating depth. I will check three times and with most bullets I get a variance of no more than .003 to .004. If it is more than that I do it three more times kick out the high and low and average the rest. Most rifles won't know the difference between .005 in seating depth.
Here are a few things I do to make Stoney Point work better for me: 1. Send a fired case from your rifle to S.P. (or Hornady) and have them drill and tap to fit the gauge. 2. Chamfer inside and out of modified case neck making sure it is very smooth with no burrs. 3. If using rod guide when you have finished cleaning remove it and carefully insert a patch rotating it while going in and out of the throat area only. 4. Then use a mop to throughly clean and dry the chamber.
Hope this helps. Good Luck, and good shooting.
