Why would my friend not tell me she's bisexual when I'm a lesbian?

[LouisePawley]

Basically my best friend knows i'm a lesbian and she's always been fine with it, we're really close and have been best friends for years. Then, her friend that I don't even know mailed me and said that we should get together, I told him that she's straight and he said that she told him and loads of other people that she's bisexual. Obviously she knew I'd be cool with it so why wouldn't she tell me but tell loads of other people that she doesn't know as well? Any ideas would be appreciated!