They don't represent all pop music. Britney and Bieber started out as teen pop and then their sound evolved as they got older and trends changed. Beyonce is R&B/pop. Pop is a varied umbrella term for music that has a certain song structure. In the '50s and '60s, girl groups like the Supremes (I personally prefer the Chantels, but that's me) were pop music (that was before the age of Autotune and you actually had to be able to sing, lol). In the '80s, Cyndi Lauper, Madonna, Michael Jackson, Prince, Blondie, the Go-Gos, etc. were pop music. The Rolling Stones, David Bowie, Whitney Houston, Bjork, Sia, etc. are also pop musicians. There's also Lana Del Rey, Lorde and Marina and the Diamonds.