A lot of thirteen-year-olds, I've noticed, generally tend to think they're much better than they actually are. Ana's comfort zone is being around people her own age, because a lot of them gravitate towards her because of her wealth. She hasn't interacted very much with the older kids so this would be an entirely new thing for her (though she's got Christine by her side). And I think that with the way other characters have powers, blood drinking would come off to Ana as kinda gross (Diancia is the only character I can see so far having a similar trait with gaining more nutrition from meat).