I spent quite a bit of time thinking about the recommendation algorithm and (after falling into a death metal hole I can't seem to escape on Spotify) I came to the conclusion that their analysis of the audio content of the song is way too shallow. You listen to songs because they have a common harmonic structure, or rythm, but not necessarily the same spectrum. That's why a metal fan might dig a cover of Metallica by…
So let's say for example there are 1,000 people who like Spears and Pantera (specific songs). Now take those 1,000 people and compare the other artists and songs I am listening to. Let's say, of the 1,000 people who like Spears and Pantera, 680 listen to a specific song by Creed. That's a high hit rate, which means that song should be offered to me as I will most likely enjoy it. It doesn't matter that Creed doesn't sound anything like Pantera or Spears.