I would add that, once your brass has been through a few firings the brass will gradually lose it's malleability. As the brass in the case neck hardens it becomes more resistive to the bullet and that increases the amount of pressure needed to seat it. Even with annealing there will often be some difference in a case neck's resistance to seating the bullet. Unless you keep your brass very well sorted into sets based on number of firings you will eventually find that bullet seating pressure varies to some degree within the batch of brass you're working with. The bench rest guys get all worked up about bullet seating pressure but, unless you're trying to put every shot in the same hole, I wouldn't worry too much about it.