The simple answer to why you feel this was is that sexual orientation is way more complex and varied that a lot of us assume. It's also common for people who are attracted to multiple genders to not feel the exact same way about each gender. Heather offers a really good explanation of that here: https://www.scarleteen.com/article/advi ... _and_women
Too, as we go through life, we may get information about our patterns of attraction that helps us see our sexual orientation in a new light. It sounds like that might be what's happening for you, and that's totally okay!
Does all that make sense?