Why do girls act like they're bisexual when they really aren't?!?

U know those stupid girls who act like they're bi but they really aren't.why do they do that? One of my friends sisters is acting like she's bi and dating this other girl who is just pretending.[ I know for a fact they aren't because they've both admitted to me they don't really like girls. ] I mean really what's the point. ? I am actually bi and have known for a long time[ I like girls more than boys ] and I just think it's dumb to act bi ??