Commentary for Kiddushin 127:17

Abaye tries to harmonize the mishnah with the baraita by contextualizing both. The mishnah refers to a case where the husband is not presumed to have brothers or sons. He can make a statement such as “I have sons” because that will not make her liable for yibbum. But he cannot change the presumption to make her liable for yibbum by saying “I have brothers.”

The baraita refers to a situation where we thought he had brothers but not sons (making her liable for yibbum). If he is lying in order to exempt her from yibbum, we could ask why he would tell such a lie. He could, after all, just divorce her right before his death and thereby exempt her. Therefore, he is believed to exempt her from yibbum.
Rabbi [Judah Hanasi] would hold that when he said he has no brothers at the time of his kiddushin, he is totally believed, because why would he lie. This overrides the presumption that he had no sons, and now exempts her from yibbum. He will then not be believed later to contradict his words by saying that he does have brothers.
Rabbi Natan says that “why would I lie” is considered like a presumption and one presumption cannot cancel out another. So he was not believed to say he had no brothers at the time of his kiddushin. At the time of kiddushin we maintain that he has brothers and no sons and he cannot later contradict himself at the time of his death by saying either that he has no brothers or that he has sons.