I can't see how brass can make a difference ... the bottom of each case sits on the same shell plate and the shoulder of each bullet contacts the same seating die.
The only way to get a variation in OAL is for these surfaces to vary their distance apart (assuming the stroke ends at the same place each time). Say, the shell plate is wavy or moves on its spindle, or the spindle is canter, or the die moves up and down in its toolhead.....right ??
Ray