Could this be caused by people with higher IQ being more honest and knowledgeable about things?

The average person has undiagnosed issues. They may have a mood disorder, but either deny that they have something wrong with them or have never bothered to have it checked out?

Compared to people who care enough to officially test their IQ. I argue that more people in a group like Mensa are more aware of their diagnosis' than the average person.