If you interview 1,000 software developers at random (which is probably about the number I have), you discover that maybe 100 of them are competent, and maybe 10 are actually great.
The 100 who are competent tend to know they're not great, and the 10 who are great often believe they're one of the 100, and the remaining 900 often believe they're one of the 10.
Anyway, that's LinkedIn explained.