Fandom growth can be based on old discography (or even something unrelated to music at all)
So it can be due to the hit cb or old content.
That's why when your fandom is big enough, sales no longer reveal whether the song is good or not.
[ Except ] groups that are debuting or in their rookie period... sales really depend on their discography
I think the only metrics you can use to see how the song is doing are :
-gp taste
- virality of the song
- radio play
- and maybe (unique listeners)
Charts and sales...both show success, but you can't use one to prove the other