Medium shares in their help article about how earnings are calculated reasoning behind all that you question. Such as true earnings can take up to 30 days to fully calculate. Additionally, you can be paid for read minutes of a member when they read your story before joining, after they join. You are also paid any individual members share of membership for the time they spent reading your article compared to others. So sometimes if a member only reads one or two stories in a month, you might get 50% or more of their pool for that month.

I still think Medium would be better off going with a single flat rate per minute. Which they calculate daily based on number of paid members that day. Would solve a lot of questions like you pose.