I am assuming in a case of dinei nefashos (DN), the reason why there is no miut, is because we are dealing with whether the defendant actually did an action that is chayav misa. We know the action is chayav misa - we just don't know if he did it. And there is only one reality: either he did it or not. In case there is no miut that is taken in consideration because there is truly only one reality. This is not the case in dinei mamanos (DM). In DM, we know what the defendant did this act, the question is if it's wrong or not. We say divrei Elokim chaim, and there are many true answers. So, in that case, the defendant can just hold Kim li - "I hold like this opinion who says it's not wrong."
What happens if we have a case in DN where we know the defendant did this action - the question is it chayav misa or not? For example, I heard that Mordechai in the story of Esther didn't bow to Haman because he was choshesh for a chumra that it might have been avoda Zara. In that case, which is DN, can the defendant claim Kim li even though rov is against him?
Soheil Zaman, United States
Tosfos 27b DH KM'L writes that what we mean when we say that we follow the majority in DN, is that we follow the majority of the judges. The reason is because the miut is considered as if it does not exist. Maharatz Chiyut writes that this is because the minority of the judges must, according to the Torah, say that the majority opinion is the important one, and that their opinion is insignificant.
So this is a somewhat different idea. We have a dispute between the dayanim and because the majority side one way, this means that this becomes the Torah's opinion on the matter.
KOL TUV
Dovid Bloom