Duh! She might as well say, "She drinks water, I eat food"! The line clearly seems to be an attempt at drawing a contrast. But she tells what the other girl wears below the waist and what she herself wears above her waist. No clear contrast there. One can easily wear both simultaneously (As I myself have done not a few times). So how are we to hear this as a contrast between the other girl and herself? The only way I can see is to suppose she means to be describing the TOTALITY of what the two wear. But I don't think that's the intention. Can you imagine Taylor Swift attending school wearing NOTHING BUT a T-shirt while some classmate wears NOTHING BUT a short skirt. If that's the image we're supposed to get from these lines too many people I know would be forbidden by their parents to listen to Taylor Swift. And barring that interpretation, the line becomes about as nonsensical as saying, "But she breathes, I sweat" or "But her heart beats, my guts digest" or so on!