User blog comment:Apocaylpse06/Ichigo Kurosaki's (New) Zanpakuto/@comment-1975828-20110815110510/@comment-28861328-20110815115645

The one that cam from Rukia was a result of her own power being forced into Ichigo. It was not his own power, thus it was not his own Zanpakutou. When Byakuya took Rukia's power from him and he gained his own Shinigami powers, that is when his soul gave birth to Zangetsu.