I don't think it's worth rationalizing why you liked girls. It's natural to be jealous of the other gender, aswell. Guys are a bit more secretive about it than girls, given societal pressure, but deep down I think everyone thinks their opposite gender has it better some days.
Also, your brain evolves, no opinions or thoughts should be taken for granted, we're always learning and we're always changing. I don't think anyone is born knowing they're trans, it's something you identify with or you don't, and it can strike you at any point in time...
Perhaps you were jealous of girls, but maybe you just happened to like them too? I don't know, it seems more common than what people want to lead to believe. =)