This is another question on Quora.com. The answer is quite straightforward. The cat you are petting is much smaller than you. The cat regards you as another cat and she wishes to be friendly towards you by rubbing against you, merging scents, head butting your head and perhaps touching noses. These are all friendly forms of cat behaviour between cats.
Your cat cannot do what she wants to do because you are so much larger than her. Your head is out of range. The only part of your body which she can rub against is the lower part of your legs. Cats do this sometimes; they rear up on their hind legs and stretch up in an effort to interact with their human as best they can, in a feline way.
Incidentally, I don’t think that your cat is necessarily trying to reach your face. He or she is simply trying to interact with you at the same level, at the same height. It may include nose touching and rubbing against you. However, domestic cats do not usually like to be face-to-face with a human. It can be a little bit disconcerting for them because our heads are much larger than theirs and we are generally so much larger than them. We are at least slightly intimidating.
There is nothing more to add, to be honest. It’s about relative sizes and the cat trying to interact, in her way, with a ‘domestic cat’ friend who is abnormally large!