Depending on how it goes, Nodoka's blackmail of Yumi will fail. She also doesn't seem to anticipate Futaba's reaction to what she does if I recall correctly. I think there is a level of omniscience in relation to the quiz, but that's more for laughs than anything. Story-wise I think Nodoka can be wrong a lot of the times, especially in relation to people's reactions, she just doesn't really care.
Even the book is a good example, it was never about getting everything right, just for it to be similar enough that her objective is accomplished. I would assume the same thing is true in Io's case, she just took a shot to what would be the most fucked up possibility that would explain her behavior. Even if she missed, some damage would be done. There is something that she says about not revealing sexual assault without a reason or something just before though, I can't recall for sure - so I'm curious about that part.