Why? The wife and daughters of a Kohen (priest) are usually allowed to eat terumah (priestly offering). But if they were violated by a non-Jew, they are considered to be a zonah (woman disqualified by forbidden relations). At that point, they are not allowed to eat terumah anymore, even if the violation was against their will. She was raped; she is not allowed to eat terumah anymore.
So this Mishnah will talk about a Kohen's wife or daughter who was abducted, who was kidnapped by an Oved Avodah Zarah (idol worshipper), and we don't know whether she was raped or not. The Mishnah says: “A woman who was captured by Oved Avodah Zarah is allowed to eat terumah,” because we don't have any suspicion that she was raped. This is the opinion of Rabbi Dosa.
The Chachamim (Sages) say: “There's a type of captured woman who is allowed to eat terumah, and there's another type of captured woman who is not allowed to eat terumah.” According to the Chachamim, it all depends on how the information that she was captured became known to us. In other words, the Mishnah elaborates: “How so? A woman who said, ‘I was captured, but I was not violated. I'm pure,’ is believed.” She's allowed to eat terumah.
The mouth that prohibited her from eating terumah is the same mouth that allowed her to eat terumah. In other words, if the woman said nothing, we would not have known anything about her being captured. And the chazakah—the assumed status—was that she would have been allowed to eat terumah. There's no evidence. There's no reason to think otherwise. We don't know anything about the fact that she was captured. So if she kept quiet, she would have been able to eat terumah.
So, by the fact that she told us in the first place that she was captured, which is prohibiting her, in a certain sense, from eating terumah, it's clear that she's telling us the truth. And then, when she right away told us that even though she was captured, she wasn't violated, that allows her to eat terumah, and we believe this woman.
And if there are witnesses that tell us she was captured, and she says, “No, I'm pure,” we say, “No, she's not believed, and she's not allowed to eat terumah.” In this type of case, when it was already known that she was captured, you don't have that migo—that possibility of her coming with a different claim. It was already known to us that she was captured. So, by the fact that she's now trying to allow herself to eat terumah, there's not enough for her to prove her innocence.
If she kept quiet, she wouldn't have been able to eat terumah. She would have been assumed to be problematic. So she doesn't have that migo. She doesn't have that chazakah that she would have been allowed. So we have no reason to believe her here when she says that she was not violated.