People are born gay, straight or bi. But if they're bi, and it's very common for people to have at least some bisexual proclivities, then they can choose whether to act on the homosexual side of their bisexuality or not.

Some bisexual women only date men. Some bisexual women only date women. Some bisexual women just fall in love with someone of whatever gender and then end up being monogamous with them and never act on the other side of their sexuality. So if you're bi, your sexual BEHAVIOUR (not orientation) can indeed be environmentally influenced. For example some bisexual people might be conservative and religious, and think it's sinful to act on their homosexual desires. So they don't ever act on it, and instead only act on their heterosexual desires. On the flip side, some "political lesbians" are actually bisexual women who decided to not act on their heterosexual desires, because they were radical feminists.

So it's possible for bisexual women to decide to only date women. I don't think it's very common though, especially not enough to account for all of the statistical differences indicated.

Other factors that go into this would likely be that younger women are more likely to date older men, but young men are not likely to date older women.