In other words Joni with a lot of casting experience and a smaller frame might need a 4 on a particular rod and a new inefficient caster weighing 250# might need a 5 just because of the way the rod is loaded.
HEY! quit pickin on us fat kids! (besides, we're more likely to need an 8wt to learn how to cast so we can feel it load. well, I did anyway)