Why Ayu using "boku" ?

[mimi.chan]

I was wondering that a long time ago.... since I haven't lived in japan I don't really know how it works, though I'm japanese... XD
but I believe "boku" and "watashi" can be used both female and male....
in my point of view i think when you are talking it refers to "watashi" to girls and "boku" to male...
but when you are singing I think "watashi" is a formal way and "boku" informal...? I'm not sure.... but that's just what I believe b/c I hear male singers singing "watashi"