User blog comment:Silver-Haired Seireitou/Sake Time with Sei - Parade of Babelicious Zanpakutō/@comment-1493741-20140531045243

I always worked under the assumption that as he forges ALL Zanpakutou and knows all the intricate processes to bonding with it, and being a complete lech, he just forged a shitload of Asauchi and bonded with them causing them to take on personalities and forms akin to his Soul. They differ somewhat, because as the omake's pointed out, Zanpakutou and their spirits can be modified.