[silenceisntgold2]

Actually, bump to this topic, but I just thought of a possible explanation, it's possible that the katana is just a piece of metal(spirit metal?), but the spirit of the zan is with the wielder, so example, Suzumushi is Tousen's spirit, but the physical sword belonged to the girl. Just like Hyourinmaru was calling to Toshiro when he was a kid. It was a voice he heard, but he didn't have the physical form yet.