I think that it is something that we may never have any true insight into that could cause this.

Since it would appear that Medium is calculating earnings daily, my assumption is they are dividing the amount that they pay to writers from each member into a daily amount. Subsequently, they then divide that amount between what that member has read each day.

As an example, if Medium decided that they would pay out $3 of the $5 to writers. They then divide this by the days in the month which would be 10cents per day. Which is then divided based on the minutes they read for the day. So if they only spent time reading a single poem that 10c would go to that individual author. On the other hand if they were to spend an equal time reading 10 stories, each story would get 1 cent. So if this were true it would depend on what sort of readers were reading each poem.

I hope that makes sense. I just do not see any other way that they could do it and report the earnings daily as they are.