It's all of it. In the past, were here women who liked other women, or were bisexual, that would have been interested in exploring that if society didn't fucking punish that kind of behavior aggressively? Absolutely. In the past those women kept that shit to themselves, at least more often than now, because there was a legit chance they could end up in an insane asylum if they got caught. At the very least ostracized from friends and family for being heretical sinners. It's not just that they'd have been "identified" as straight, they'd be pretending to be straight for their own survival, and all that it entails. Same as gay men. Sure, it's always existed. But not it's not punished by the literal laws of society.
And because society has become more open and accepting of same sex relationships, guess what, both sexes now have a theoretically larger pool of people from which to choose, at least if they're comfortable with both sexes. Personally, I'm not. But I'm not every person on the planet. You can "reject" the idea of women choosing women because they think men suck all you want, but there's an increasing drumbeat of those stories being told. In the past, women who just got the shit beaten out of them were almost legally forced to stay in those marriages. Now? Plenty of women who say "fuck me, I'm done" and guess what, they can peace out and find a relationship with someone they feel safer with, regardless of gender. Sure, women who like dick still get dick. And maybe they get married, but maybe they don't because that's no longer a required societal structure anymore. It all adds up.
And that's before you even factor that women are going to college at greater levels than men these days, and there's a growing segment of incels and shit. Maybe some of those women at the top end up alone. Maybe they choose women. It all adds up into a not statistically insignificant number.