It's fairly simple and the "mythical algorithm" is anything but. Seriously, by now it's an urban legend...

Anyway, the explanation is this. An article that does very well will typically attract occasional readers more. Occasional readers generate more $ because their monthly $5 is split up by just a few reads whereas those who read a lot end up spreading that amount over a much larger pool of articles. That's one aspect to keep in mind.

The other is read-time. You see, what you're looking at in the graph is read-count. Now, imagine if everyone spends just 5s per read reading, you could have 1000 reads and would still only amount to barely 1.5 hours of read-time. You'll see that the truly successful articles have longer (closer to their actual length) read times, and more engagement because frankly it's very hard to comment on an article if one hasn't spent at least 20-30s reading. Hope this makes sense and clarifies things.

I wrote a helpful article recently about some other often misunderstood aspects of Medium, give it a read, maybe it helps with other questions you might have: https://medium.com/illumination/everything-you-got-wrong-about-medium-51ab50c125ab