And yet every manager I have ever had asks me not to discuss salary with other co-workers to prevent them from getting jealous. Usually in the same discussion where they tell me I am the highest paid one.

This is usually a few weeks before I have an offer for 40% more.

I think they lie because they know they are going to lose me anyways so it's a gambit with a low probability of success, but no additional penalty for failure.

I've been told this and I'm pretty sure it was true, and I was underpaid, and everyone else was underpaid worse.

It's a lame argument even when it's true.