GCSE R.S. is biased? Watch
Learning and remembering all those stuff - they've had to do it as well, just at an earlier stage. We have some atheists in our class that comes out with 9s all the time. Similarly with language, you can come out with a 9 if you're not native, it just requires extra work. In fact, native speakers may struggle getting 9s, especially in writing and speaking, because they write/speak naturally, not in a regimented fashion, which can also be applied to RS - they know the teachings and practices, but when talking about it, they won't expand it enough.