Sexuality can be visualized like a dial. Sometimes the dial points closer to one side than another. You're simply more sexually attracted to girls than boys, and there's nothing wrong with that. Someone can be 5% attracted to boys and 95% attracted to girls, that doesn't make them invalid or any less bisexual. You are who you are!
Many people would say that sexuality is a spectrum, a continuum rather than a very discrete, black-and-white concept. You might find some more information by looking up the 'Kinsey Scale' to read up about how scientists have tried to break down a person's orientation. It's completely natural to have sexual attraction and it makes you who you are.
Not everyone who identifies as bisexual has an even attraction to males and females. I've heard people describe themselves as "lopsided" or uneven bisexuals because they like both males and females but have a stronger preference for one over the other. It could mean that you're bisexual with a stronger attraction to female, it could mean that there are specific situations or factors that lead to your attraction to males. It means you're perfectly normal!
That's ok, you're simply a bisexual person who tends to be more attracted towards one gender, it can happen and it doesn't mean you're not bi! Being bisexual doesn't necessarily mean you're attracted 50% by guys and 50% by girls. Whatever the "percentage" is, you're totally entitled to identify as bi!
What you are experiencing right now is totally normal! The way you are feeling right now depends on several factors such as the completion or not of your sexual maturity as well as the characteristics(both internal and external) you find attractive in both girls and guys. In no way should you consider this "imbalance" in your sexual preferences as a sign of confusion of sexual orientation. What is clear is that whether consciously or /and subconsciously you're noticing traits in girls you consider more attractive!
Always keep exploring your sexuality and never stop trusting what your heart tells you!