Sadly, there are a lot of girls who think that saying that they kiss girls will mean that boys will like them more.
It's extremely pathetic and sad, and just makes it all the more harder for those who genuinely feel that way cuz they end up being lumped in with these saddos.
Also, a lot of teens go thru phases where they might have crushes on the same sex, but may end up being straight. That is why there is no point whatsoever in a thirteen year old girl proudly proclaiming that she's bi, just cuz she's crushing on her maths teacher or cuz she's kissed her best friend. In a few years, she might have gone thru the phase and out the other side straight.
Finally, some immature little girls have a tendency to copy their favourite celebrities in everything they do; remember that Britney/Madonna kiss? They see one of their idols kissing other girls so they feel they have to do likewise.
When, eventually, they grow up, they won't feel the need to copy celebrities so much and will hopefully get a life of their own, not one that is just a poor imitation of Britney Spears, for example.