How does scoring work if (allegedly) 10% of questions aren’t scored?

Sorry for the confusing title, but I’ve heard 10% of questions you do in the ucat don’t contribute to your overall score, but how can this be the case if, for example in QR 36 is 900 but 35 is around 880? What if the question answered incorrectly was of the ones that don’t contribute to your score would it still be 900 despite an answer being wrong?