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) are most valued. To rephrase colloquially: if you aren’t confident about your skills/accomplishments/whatever, why should the person making the hiring decision be confident in them, when they know less about you than you do?
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) are most valued. To rephrase colloquially: if you aren’t confident about your skills/accomplishments/whatever, why should the person making the hiring decision be confident in them, when they know less about you than you do?