> (Sometimes at an astonishing penalty to their earnings...)
Which is the primary reason why earnings are typically kept secret. Company policy almost certainly prohibits you from saying: "John makes 110k/y Joe, you are three times as good as John and you make 90k/y, we most certainly are not going to pay you 330k/y, but maybe you should ask for a 30k raise ?"
One of the more interesting things I read about is how Salesforce.com reviewed their employees and their salaries, realized that female employees were underpaid, and increased their salaries accordingly, rather than waiting for someone to ask. I'm afraid that's rather rare, though.
There may also be some distrusting "Why is my manager encouraging me to leave? I'd better dig myself in and make sure I don't fuck anything up" thoughts (although I'd like to think I'm not thought of as "that guy").
But I also see cases that are closer to a reverse Dunning Kruger effect, people who just don't realise (or admit to themselves) how good they are compared to their peers. (Sometimes at an astonishing penalty to their earnings...)