User blog comment:Marijuanna/Rare Zanpakutou/@comment-3177945-20130117165953/@comment-5847836-20130120001619

Well, Ichigo's was originally in an Asauchi-like form. Possibly something in a Shinigami's power naturally manifests in that fashion if they have enough power; in essence, he, Toshiro, Zaraki, etc. did unconsciously what Oetsu did by design? This would be in agreement with the earlier explanation that it is the result of focusing and solidifying the Reiryoku.

Also, Asauchi presumably begin all identical, but after attaining Shikai and again sealing, the form changes shape and is unique to the Shinigami, being a reflection of their soul.

That would explain how Shunsui now has a daisho, Soifon has a wakizashi, Iba has a tanto, etc. It simply reflects how they fight best.

And as for Starrk, Lilinette is apparently the physical form of his Resurección. Like the other Arrancar, some of his Hollow powers were sealed away, except his took the form of a living being instead of a Zanpakuto. Recall that Starrk was a natural Arrancar, so we don't know exactly how that happened.