Luna has felt this way as long as she can remember. The few people she tells who understand the initial admission tend to then stumble at that part, since they have an image in their minds of some great epiphany, but to Luna it has always been as natural as breathing. It isn't something she became as much as it is something she is, the same way Harry did not have to discover he was a boy and Ginny never felt surprised to learn she was a girl. (At least, to Luna's knowledge, though assumptions can be dangerous.) Most people don't even get that far, and look at her with confusion if she ever brings it up. Some of them look at her in ways that are much nastier, but Luna supposes that's a type of confusion as much as anything.
"I don't understand," Ginny says predictably, brow creased. Her mouth twists, knotting up at one side in a way that borders cute and ugly and that Luna labels 'endearing'. She has the careful look sometimes worn when Luna is around but not, Luna suspects, when she isn't. A difficult hypothesis to confirm, but probably true anyway. It's the look Ginny gets when she sorts through and picks at what Luna says, looking for the real meaning. It's quite a nice look, because it's one which says Ginny doesn't want to fall prey to her own pre-conceived ideas, which Luna has always thought is a valuable trait. It's certainly kind of her to try, and it's far better than an outright dismissal. Better than, 'that's impossible' or 'you don't know what you're talking about'.
Even if Luna doesn't understand why it seems to be so complicated for everyone, Ginny included.
"I'm not a girl or a boy," she repeats patiently, "-or anything else, for that matter. It's not very difficult. In fact," she adds thoughtfully, "I've always thought it was probably a lot simpler than being any of those things."
"After all, if you aren't a girl or a boy, then you don't have to worry about whether you act the way people expect you to. Nobody expects anything of you. And you don't need to know what it is that makes you this way or that way." Luna taps her chin thoughtfully. "I don't think anything made me any way. I just am."
"Yes, you are, Luna," says Ginny, in a tone of voice which says she doesn't understand but doesn't mind not understanding. "Nobody's anything like you, that's for certain."
It's a nice sentiment. Luna tucks it away, and knows that it's true, and smiles happily.