I think we're assuming that the many users who listen to indie bands spread their listening among many different bands (each use has a difference preference) whereas users who listen to top 40 all listen to the same artists.
There is also the idea that some "users" are businesses playing background music all day, and that they are more likely to all choose the same pop songs (which the most people are familiar with) and to play them all day everyday. Unlike a normal user who typically listens to a wider variety and for fewer hours a day.
There is also the idea that some "users" are businesses playing background music all day, and that they are more likely to all choose the same pop songs (which the most people are familiar with) and to play them all day everyday. Unlike a normal user who typically listens to a wider variety and for fewer hours a day.