If one is compressing the load then there will be variances in seating depth. The powder will not settle the same in all the cases.Don't understand how neck work hardening would affect OAL. If you assume the bullet seating stem tip slippage isn't involved. Your die is set to a certain fixed position, the case is pushed a fixed amount into the seater based on how you adjusted the die. When you pull the press handle all the way down there should be no variation on how far the bullet is pushed in. The leverage of the press will overcome any variation in neck tension. Are you measuring base to ogive or base to tip? The tip closure looks different on almost all bullets.