Why do you think so? I would personally like more people who are actively talking about their good and bad sides, although I'm not sure if I'd do that in an interview, because it might mean they don't know what appears to be the most effective strategy.
I think, just like you have been recommended to assume the person conducting the interview is not rational, that person has as a default you are not rational. To expand: in general someone filling a job position will have many applicants. They will go through a negative search procedure: looking for ways to quickly discard applications. Since most applicants are overconfident, sounding less confident means you are perhaps less skilled than the overconfident applicants. Furthermore, employees who are confident about their deadlines (and meet them of course)...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.