Part of it is are they Medium members or not. More Medium members means more money. Unless they are not members and decide to become Medium members using your sign-up link, in which case, you get a LOT more money. (So far only one person has done this, but I am very grateful for that person!) Then there’s how long they spend reading your article (which is why short forms tend not to make much) — which includes the time spent reading the comments.

As well as various and sundry “adjustments” made by Medium behind the scenes, just to keep everyone guessing. :-)