Seating pressure can also influence seating depth consistency. I first noticed when I was loading the same bullets into cases prepped the same way but doing an increasing amount of powder; example 30-06 loading RL22 in sets of 3 at each powder level, 59.0, 59.5, 60.0, 60.5 etc.
When the bullet starts compressing the powder, the OAL would increase. IMO this is due to increased pressure on the linkages of the press and the seater can not seat as far.
Also happens when you have an inconsistent neck ID or inside neck surface.
IOW when a bullet is harder to seat for whatever reason, the OAL will increase slightly
