User blog comment:Xilinoc/Ch. 664 - The Gift/@comment-25897574-20160228212945/@comment-11051098-20160302010504

Slaying Hollows may be the original purpose of Zanpakuto, but perhaps some Shinigami just have a potential so great that some Zanpakuto just end up becoming way more powerful "by default".

Yamamoto was the strongest Shinigami the last 1000 years, and he was more than 2100 years old. Maybe Ryujin Jakka became as powerful as it was because of his potential being that massive. Also there's the fact that Ryujin Jakka was at least 1000 years old and its Bankai state basically could be called the definition of "lethal" at the time of Yamamoto's death.

That's my theory anyway.