As per DP, it was Chaya-Sati who had entered the fire. Sati had already left to her Loka leaving Chaya-Sati handle Dakash, after he insulted Sati. So it was this Chaya-Sati born as her Anasavatar, Ganga & Sati the Aadi Shakti as her Purna avatar Parvati. While doing Tandav, Shivji carried her body on his head, so her Anashavatar will flow from his head, that's why Aadi Shakti told him about Ganga & Parvati.😆I have DP at office so I can check & DB at home.