You might address this possibility later in the article, but my immediate instinct is this: Medium, in 2019, announced that they'd start to pay writers for the read time they generated not just from paying members, but also from people who weren't members at the time of reading but signed up within 30 days of having read a story they'd written. So in other words, if your semi-viral story brought some people to the platform, you'd see inflated earnings for 30 days or so following its publication... after which your earnings would go back to something more closely resembling the read times of people who are already members. :)

(I could be wrong, of course! Medium's algorithms definitely confuse us all. But I hope that helps.)