This may be one reason why people are reluctant to assign numbers to beliefs in the first place. People equate numbers with certainty and authority, whereas a probability is just a way of saying how uncertain you are about something.

When giving a number for a subjective probability, I often feel like it should be a two-dimensional quantity: probability and authority. The "authority" figure would be an estimate of "if you disagree with me now but we manage to come to an agreement in the next 5 minutes, what are the chances of me having to update my beliefs versus you?"