I don't disagree, but in this case they are actually the exact same thing. "Hair metal" was a derogatory term someone came up with to describe glam metal bands of the late 80s who they felt were more obsessed with their extravagant hairstyles than with actually making music.

They are literally two different terms for the same genre.

Some fans who either didn't get the joke or just didn't care I guess eventually started using the term endearingly