What is with this cat?

Okay, so a stray cat gave birth to kittens in my neighbor’s backyard. We discovered the kittens within hours of their birth and brought them in out of the cold. The mom was sweet and cuddly, although starving to death. She was probably dumped off. Her kittens seemed to be premature, and one didn’t make it. Of the other two, there was a calico that was the biggest and healthiest. The other was a tortoise shell that was small and frail. We diligently made sure the little one got to nurse. As they got old enough to play, the kittens were always wrestling and biting while playing.

Long story short, we kept the little tortoise shell and found homes for the mom and sister. I still attributed her biting to being a kitten, but now she is two years old, and still when anyone tries to pet her, she turns her head to bite them.

It is not vicious or anything, but I never saw a cat that you couldn’t pet. I try to stroke her head, and she ducks and tries to bite me. If I let her, she doesn’t bite hard, just like she is still playing.

Any ideas on how I can stop this weird behaviour?