Every time you use a knot to change tips, you use a bit of the leader to tie and cut of the remaining end. So after changing a tippet or four, your leader changed from a 4x to a 3x/2x.
When you use leader rings, you use only once a little bit of your leader. then tie the tippet to the ring. If the tippet is to short, cut the tippet at the ring and tie a new one to the ring. You never touch the leader again.