27 Iyar (Shavuos)

Yerushalmi Rosh Hashana Perek 4, Halacha 8 says that the sair of Shavuos is only one not called chatas(Bamidbar28,30) because Hashem considers it as if never sinned on Shavuos because accept Torah.But if so why does Vayikra 23,19 call Shavuos' sair a chatas? If Hashem considers it as if we never sinned on Shavuos, why do we need a chatas in Shemini and not in Pinchas?