This is probably somewhat oversimplified, but I think part of it is that society tells you that "feminine" is a pretty constrained set of X things, whereas "masculine" is Y things, and society clearly values masculine over feminine, and Y things individually over X things. If you think about how media is marketed, women are expected to be interested in and identify with media about men, but media about women is "feminine," for women. So it might be easier for someone assigned female at birth who's comfortable having a very feminine gender presentation to nevertheless be extremely conscious of how they interact with gender and to get to the point where they want to say, "I'm non-binary, not a woman," than it might be for someone assigned male with a very masculine presentation to say the same thing.