I thought the point was obvious, but okay, I'll have another stab at explaining it. If each member is paying $5, that means there's $5 per member in the pot to get shared around. For every member that earns more than $5, there will be a member that earns less than $5. Of course, this is a simplified version of what's really going on. But surely you can see the logic, right?

I know it's the read times that writers get paid for, but it doesn't matter whether I spend 100 hours per week reading many articles or just 20 miutes reading a few articles, the writers of those articles are only going to get a share of my $5 (minus whatever amount Medium takes for itself), nothing more. The more articles I read, the less each writer gets from my read time, because my membership money is finite.