The trouble is that when they create the animations for the musicians, they hard code the animations for the song. Which can also be fine, and means that the musicians move exactly the same way every time you play the song.
The trouble happens when you combine those two things. Since you can have any character play any instrument on any song, you get some really strange things going on. Here's some examples from Youtube.
Kurt and Johnny Cash both love them some Elton John. Did you know that Cash can wail on the drums, or that Kurt only played lefty by choice?
Maybe I'm just old-fashioned, but there's something very strange about seeing Cobain in something like this game in the first place, and even stranger seeing him miming Flava Flav.