[pessimizer]

I think the "effect" is simple statistics. If you know more about a subject than most people, fewer people will impress you with their knowledge when talking about that subject - this is fairly definitional (with a little dithering about "most" and means and medians.) Additionally, someone who is very good in a lot of subjects could be expected to be worse in a lot of other, unrelated subjects (assuming a limited budget of study and attention.)

I think this is only shocking to people who believe in a general g that simultaneously makes people smart in everything. There isn't a person alive that doesn't know enough facts that I don't know to take the rest of my life to enumerate, even if it has to get down to the level of knowing that their grandmother likes raisins in their oatmeal.

The closest we have to g is reason, and people whose reason is rigorous have rigorous reason everywhere that they aren't being willfully ignorant (like declaring that normal reasoning doesn't apply when it comes to X.) I'm not ever disappointed in the intelligence of people I know with good reasoning skills, unless you count groaning when I realize how long it's going to take to get them to speed on something I have a lot of knowledge about.