I personally think that By separating albums into rock, pop, R&B, Metal, or Rap, the best-sellers may be dramatically different numbers. The result is certain genres that have lower grossing albums naturally. The bar is lower. An artist like Pink comes along that can arguably be in either pop, punk or rock depending on who you ask. Her sales records would vary greatly as far as rankings from punk to pop.
Just because the album says best selling it doesn't mean that its a good representation for the musical integrity i mean the album could be Justin beiber and most people consider him to be a bad pop singer and he has sold a lot of albums do to the crowd that would buy his album.
Although music may be pleasing to the ear, that does not necessarily mean that it has integrity. There is a lot of really bad crap including the random noise from certain genres of music that have no musicality. For some reason that is popular among young people, but it really has no merit. It's noise.
The term "Best-Selling" means nothing. A record doesn't even have to sell very many to be labeled as "Best-Selling". It's just another tool used by marketers to sell more records.
Just because a lot of people like it doesn't necessarily mean it's good. A lot of people smoke, yet we know that is bad. I will also say that the majority of the public doesn't have great taste anyway. For instance, Fast5 was #1 at the box office for an amount of time.
Not everything that is popular is good. And not everything that isn't popular is bad. Everything, especially entertainment, is a product of a majority of people thinking it is something worthwhile. Just because an album may be best-selling doesn't mean that it is any good or possess any musical integrity.