That's a great observation. For what it's worth though, it does seem logical to me that the best would also be best at estimating their skill. Not necessarily because they're better at it per se (though there's likely some of that too, for the reasons originally posited by D-K), but also because they have an easier problem to solve. When you know something well, it's fairly obvious that that's the case. (Think of the experience of acing a math test. It's entirely possible you'd know you answered everything correctly.) When you struggle somewhat though, it's much more difficult to estimate how much you're struggling compared to how others would fare.