From what Medium said in their recent emails, they look at how much you receive in Partner Program payments for the month in question, and if you're in the top 1000 you get $500, between 1001 and 1500 you get $100, and from 1501 to 2000 you get $50.
This because they use the Partner payment as their proxy to assess the member engagement you bring into the platform.
By the way, regarding the reason viral content pays so much better than non-viral articles, I believe I "cracked the code" on that, as I described here: