If you are using a progressive press, the upward pressure of the sizing, flaring, and crimping stations will change your seating depth a tad. Usually it will increase my COAL by as much as .005". When you are making your seating die adjustments, generally you are working on one cartridge at a time and the other stations are empty. So you get a little upward pressure on the toolhead in the press. When the other stations are assembling cartridges the seating depth will change with that toolhead play, but usually it goes in a very predictable direction. Usually it increases COAL, but depending on what machine design you are using it could decrease it. Either way, it is a fairly predictable change.