i think a big part of it is just that she knows that behavior isn’t normal for you. if you had always responded to headbutts with headbutts then she’d probably be fine with it, but since it’s not your normal reaction she’s like “dude why are you being weird”

also in both cases she’s probably fishing for the very specific reaction of you rubbing her with your fingers which probably feels better than a headbutt or a bite

stop biting your cat