A possibility I've never seen discussed: The Medium algorithm steps the payouts or has earnings tiers, so that you earn less with more followers, as counterintuitive as it sounds.

I've written books for major publishers, and in one case, when my literary agent went over a royalty statement, she discovered that your royalty rate went down when you sold over a certain number of books because of tiered payouts. She said, "This is nuts! You earn less the more you sell!" (because of the stepped payouts). For context: My agent is very experienced, and the incident involved a major publisher whose name you would know if I said it.

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