If you look at Velocities ( if you use a Chronograph for testing ) you can see in grouping at a certain range , that there could be multiple velocities that print good groups , at that particular range ( sweet spots ). So if you get brass that has a different case volume ( velocity could change ) , with your loading , it could be out or of the sweet spots for groups .
You just have to make notes & adjustments for that brass , it usually changes with brass brand , but different year head stamps of mil spec ammo could also change. The fact that you are fire forming the brass in that particular chamber is another good reason to sort & use the same brass for that chamber , even though you resize it (FL or SB ) SB being closer to SAMMI spec's .