Guess I'm a little uninformed...how does the headstamp affect OAL? The seating die reamins the exact same distance from the shell plate no matter what the brass is. You may have more projectile engagement into the case, but isn't the OAL the same?
just askin.............
My understanding is that different head stamps (Speer, Remington, etc...) have different case lengths.
Like you, I've wondered why the OAL would not be the same and the only thing I can think of is the longer brass touches the die and prevents the bullet from going deeper.