Why must a Zavah bring a Chatas and an Olah?
Ramban: Refer to 15:2:6:1,
Seforno #1: It is the punishment that Hashem meted out to Chavah for eating from the Eitz ha'Da'as,
for which all women must do Teshuvah.
As the Torah writes in Bereishis, 3:16.
Hence a Zavah brings a Chatas to atone for the sinful act, and an Olah to atone for the sinful thoughts (Seforno).