I think that a fairly significant number of bisexual people can subconsciously choose to ignore their same sex attraction and focus on their opposite sex attraction when they first start to get crushes. Bisexuality is even still not always accepted as real, in the 80s or 90s or before it was barely acknowledged as a possibility. And homosexuality was generally seen by even the liberal public as something unfortunate to be accepted because it can't be changed. Someone feeling attraction to both sexes just focussing on their opposite sex attraction to make life 'easier' was really common. There may also be a factor, especially among women, of only accepting their bisexuality after they have had children.

Then when decades down the line something makes them realise that they are attracted to the same sex as well, you get the myth of sexual fluidity. It's not sexual fluidity, it's just finally reaching a point where you are more open to accepting something about yourself that used to scare you or meeting someone you are so strongly attracted to that it overrides your previous defence mechanisms. It can also happen the other way around where someone who has historically been exclusively same sex attracted meets someone of the opposite sex they are attracted to.