Howard said "They usually pay at least as well as ones we get ourselves, and that's after any agent's fee, which suggests that the agent is better at his job than we are (as you'd expect)."
Or maybe it shows we are more soft-hearted and don't want to overcharge.

Mind you, I often use someone else to negotiate my fee - the band leader, if asked I say "I'll have the same as the musicians". So with some bands it's two thirds of what I get with others. I don't much like negotiating, I want people to be able to afford me for their party, charity etc. but don't want to be taken for granted (had too much of that as a Playgroup Supervisor).