The person admitting doubt may be treating the subject more seriously than the person performing certainty.

Certainty is easier to perform than to earn. Someone willing to say they are unsure has usually gone far enough into a question to reach the parts that resist an answer, while total confidence often means the difficult section was never reached. Doubt of that kind is a sign of engagement rather than weakness, and it sounds less impressive than it is worth.

When you hear two people disagree, notice which of them can say where their own case is weakest. Give your attention accordingly. And practise naming the specific part you are unsure about instead of defending a whole position you only half hold.