That graph is what the herd behavior looks like. But if you break down, take 2 people who are actually on bottom quartile. While on average they think they're 3rd quartile aka a bit above average, one of them will have impostor syndrome and the other one will think they're the smartest person around. Then for the top 2 smartest people around, one will know it for sure and the other will think of themselves less than the overconfident newbie described above.