I thought it was stated that zero squad isn't necessarily the strongest of all shinigami; but that all of them - while all captain or higher in power - had all done something to earn the promotion, like invent the swords etc.
Hence why Yama could be stronger but still not in it.
Then again, it was also said somewhere that each one of them were stronger than all the 13 squads by themselves. Although that was way before this arc, so Iunno
XI Wiki



