Nice observation. I am just taking a random shot at explaining your observation. Maybe I am wrong. But here it goes.

If Medium charges paid members $5 per month, it needs to distribute at least $4 to the authors every month. (assuming they make a profit of $1). Basically if the audience that has clapped for you has clapped a lot elsewhere also, per clap value goes down. Whereas if your audience has exclusively clapped for you or a few other authors only, then per clap value goes up. And i think there should be some logic around unique claps and total number of claps.

This means unique articles directed towards exclusive audience will get rewarded more per clap.

Secondly, when more people are clapping for you, say 100, it is more probable that many of them are also clapping else where. Which reduces their per clap share that gets allocated to you. If 1 or 2 people are clapping for you, it is more likely that these people are selective and have not clapped at other places much.

This, I think, also checks how much maximum revenue you can make out of an article.

I think in future Medium will create a still further exclusive club of great authors whose content can be accessed at a higer premium. This helps the audience and authors both. So for now publishing more & more of high quality content is the way to go. Balance between exclusive and popular.