So I think that many people found it confusing or a plot inconsistency that during the SS arc, it was mentioned that the title "Kenpachi" was given to the strongest shinigami in the Gotei 13... yet it is later revealed that there are many shinigami far stronger than Kenpachi including Aizen, Yamamoto, Isshin, Urahara, Kyouraku, Ukitake, Unohana, Yoruichi, and Gin. It made one wonder about the definition of "strongest" shinigami. Strongest in what exactly? With the new information about Azashiro Kenpachi, the 8th Kenpachi in SS it all became clear. Azashiro rightfully deserved the title of strongest shinigami. His power was at a level that even Aizen himself respected and claimed that his Kyouka Suigetsu would not work on (Even Yamamoto had to grab hold of Aizen by getting stabbed to prevent KS from affecting him). From what we know of the novel, Azashiro did "something" and was imprisoned in Mugen by SS and Gotei 13. He was not "defeated" and succeeded by the 9th Kenpachi. No, rather the 9th Kenpachi was actually just the Vice-Captain of Azashiro and was auto-promoted to captain of the 11th squad and subsequently given the title of 9th Kenpachi. Evidently this means that 9th Kenpachi was not necessarily stronger than Azashiro or even captain material really. Now you might ask, well why wasn't Hisagi and Kira promoted to Captain in the absence (betrayal) of their squad captains? Well that is because the 11th squad captain position is a special position. The 3 requirements to become a captain (as outlined by Yoruichi) does not apply to this squad. Anyone that kills the current captain becomes the next captain. Well since Azashiro wasn't killed, this ritual could not be completed which left a vacancy in the position. The next was to promote the second in command to captain. So the 9th Kenpachi did not "earn" his position. He was just given it. Being a mere VC level, it was no surprise that the 10th Kenpachi beat him and took over. And the same with the current Kenpachi. Basically it's just this: Azashiro was imprisoned and not defeated and thus the title of Kenpachi was handed to some fodder. This means that the title of "strongest" was seriously downgraded, devalued, and cheapened. The fact that Zaraki holds the title of Kenpachi means nothing. Zaraki is of course still a pretty strong captain but he is definitely not "the strongest" or anywhere close. I'm amazed at how Kubo managed to retcon this plot hole so well with this backstory.