I wonder if some of it is because we don't have an equivalent word like 'bromance' for girls so they use bisexual instead? In my experience some young women and girls can have really intense female friendships during their formative years that can almost be like romantic relationships without the sex. It's not difficult to see why in these identity obsessed times they might try to label it when really it's quite a standard (although not universal) girlhood experience. I also have a theory that one of the unconscious reasons for these intense friendships is that they can serve as a 'safe space' for exploring relationship skills that are useful later on.