It seems to me that in most media, the classification of works in various genres remains fairly consistent over time. In books, Ten Little Indians by Agatha Christie was considered a mystery when it was first published, as it still is today. In movies, Casblanca was considered a romance when it first debuted. It still is. In television, Star Trek was considered a sci-fi show, and it is still classed as such today. But it seems to me in music that whatever genre any given group, album, or song is considered might well tend to change over time.
A perfect example of this is Rush. When they first debuted in the Seventies, they were considered a heavy metal band. Over time, however, they came to be considered "hard rock" or, more often, "progressive rock." And while I do believe Rush's sound softened over time, I don't see that it changed so drastically that they ceased to be heavy metal. A more severe example may be Roy Orbison. Back in the late Fifties and early Sixties, Orbison was considered a rock 'n' roll singer. Since then I have occasionally heard him described as country. Now Orbision certainly did perform songs that could be counted as country, but it seems to me that the majority of his work were either rock 'n' roll or ballads. I think he is best counted as a rock performer who sometimes did country songs.
Of course, sometimes artists are counted in genres into which they don't even fit. I remember in the early days of Van Halen, they were counted as heavy metal. Now eventually they would be considered hard rock. That having been said, I don't see how they could have been counted as heavy metal. Even then, they sounded more like hard rock to me. An even more extreme example may be Kid Rock. I read an article many, many months ago in which he was described as a rock star. Now, quite frankly, to me Kid Rock sounds about as much like a rock performer as The Beatles do a swing band. Or The Killers do a calypso group. If I were to classify his music, it would be as rap. He has more in common with Ice Cube and Eminen than he does The Who or Cheap Trick.
Anyhow, it seems to me that to some degree music genres and subgenres tend to be amorphous and everchanging. Today's heavy metal is tomorrow's hard rock. I don't really have any explanation for why, except perhaps for the genres themselves changing and evolving over the years. Of course, I have no explanation for why some artists are placed in genres into which they do not even fit (Kid Rock is not rock). That doesn't make sense to me at all.