I think a lot of this has to do with the fact that claps and responses are public and a simple google search of someone’s name will show which stories they’ve clapped for. I think many people are hesitant to clap for stories about certain topics but clap liberally for stories that they think they SHOULD be reading.
In a community where many professionals use their full names and network with others in their fields, clapping can feel like giving absolutely everyone access to your computer’s search history.
I believe Medium pays for reader engagement in terms of read time as well. I have one article that received 3 claps from 3 fans (but plenty of reads) and earned almost $6.