I think there is a group of people that claims to be bi to be popular, and, from what I've experienced, I'd say it's mostly girls. From what I've seen, there seems to be something of a fad that teenage girls can go through, especially associated with alternative subcultures, where everything they do is about how sexually liberated they are. Everything that falls out of the conventions of heterosexual relations is so cool and edgy, and if you have a problem with it*, then you are so homophobic. I've known girls from such scenes that claim to be bi who've never had a girlfriend nor displayed any actual female-female desires apart from loudly commenting that they find <common female pinup of the month> attractive.
Obviously, this doesn't mean that all teenage girls that come out as bi are liars. Some will be genuine. Some will be confused. Others might be genuinely bi, but might grow out of the phase, as patronising as that concept would be to them. Some, as I did, will come out as bi to 'test the waters' and not commit to being 100% gay or lesbian. If you care, it's up to you to make a judgement call in your own head about whether you think they're legit or if it'll last. Please, though, don't make those concerns public. Take it from me, denying an LGBT teen when they have come out is incredibly hurtful.
Nor does this actually matter. In any civilised society, one's sexuality or lack of it is one's own business, provided one isn't deceiving anybody or abusing them. Claiming to be bi for the social status of it is a bit silly and childish to my mind, but doesn't actually do any harm. Nor does it matter whether or not one can or can't change one's sexuality. It doesn't matter why one is / is not attracted to whatever particular genders one is / is not attracted to, unless there is a problem with one or some of those configurations - and there isn't.
* The problem I had with these girls in particular is that I was suddenly extremely popular when they found out abut my sexuality and yet they weren't that interested in me before - I loathe the thought of being 'fashionably gay'. Obviously I'm not homophobic.