I think a problem might be that even if you have an internal rating system, it's still a spectrum and the other person doesn't know what that percentage of confidence you have is. If you're 95℅ sure, maybe you still want to communicate that it isn't 100℅ since assuming you're 100℅ sure might cause problems.

Things change all the time, even if it's part of your field of expertise. Today, gorillas are technically monkeys and Pluto isn't a planet, but lots of people are sure those aren't true.