I agree Kenpachi could be listed in the high group if he actually took fights seriously, but he doesn't and it is an inherent personality flaw that lowers his combat ability from his theoretical max. Also as stated before recently I agree captains can improve themselves and experience plays a large role. Toshiro might be the reincartaion of some spirit, and be a genius but he is still inexperienced, Byakuya is more experienced then Toshiro but still no where near experienced as Yamamoto, Shunsui and Unohana