Interestingly, although Medium was clearly trying to be more transparent in how recipients were selected, the exact wording was somewhat vague:
This past month, you were in the top 1,000 writers in the Partner Program. Congratulations! To celebrate your achievement, we’re rewarding you with a $500 bonus.
When we awarded bonuses in April, we heard from writers that the criteria for qualifying wasn’t clear. We listened, and for May, we simplified how we picked. Now, bonuses are based solely on the top writers in the Partner Program in a specific month.
Medium members are drawn to thought-provoking writing from a diverse array of voices. So to remain a top Partner Program writer, we recommend regularly publishing unique stories for members to explore, read, and love. In other words, keep doing what you’re doing!
– Your friends at Medium“