Great question! The Rishonim ask a very strong question on the Gemara on 87b. How is it that Rami bar Chama -- who seems to think the Mishnah is discussing a Shevu'ah mid'Oraisa -- does not know a Mishnah in Maseches Shevuos and makes such a "mistake"?

There are several answers. The Shitah Mekubetzes offers that Rashi is trying to answer this question. He says that Rami bar Chama did not think for a moment that the Shevu'ah in the Mishnah is actually mid'Oraisa. Rather, he thought that im is not an ordinary d'Rabanan, but a Shevu'ah that is like a d'Oraisa, so it cannot be turned over onto the husband.

According to this, it is understood why Rashi here does not bring the Nafka Minahs he brings later, where he is distinguishing between a real Shevu'ah mid'Oraisa to one that is mid'Rabanan.

Kol Tuv,

Aharon Steiner