User blog comment:Silver-Haired Seireitou/Sake Time with Sei - Three's a Crowd/@comment-1493741-20150517200014

Hm.

I have two ideas at the moment.

The first is that a Zanpakutō takes on a piece of its wielder's soul. Not the whole thing, they share the traits of their wielder to some extent, but are very much their own beings. While they have some things in common, they obviously develop in different ways from their masters. So, one possibility is that a new Zanpakutō would take on a different piece of their wielder's soul and absorb different traits as a result. Their abilities would probably differ but share similarities to their sword-sibling. Yamamoto would always have Fire-type Zanpakutō even if the utilization of them is different and so on.

The second one is... Katen Kyōkotsu. Its perhaps the most unique of the sealed Zanpakutō in the franchise, and this could be the reason for it. Perhaps the Zanpakutō is always the same, no matter how many Asauchi you imprint with your soul as you theorized above. But maybe the effort isn't fruitless and that doing so is a sort-of pseudo-reforging of their current Zanpakutō. The Asauchi is assimilated and that second piece of soul merges with the already gifted piece in order to empower the lone Zanpakutō resulting in things like Katen Kyōkotsu.