It stands to reason that a school with highly competitive entry to Year 7 will have excellent exam results. Anything less would be of huge concern in terms of pupils reaching their projected targets.
However it also makes it harder for such schools to demonstrate value added and gain credit for these results with Ofsted. Taking 180 very able students through to great GCSE results is far less of an achievement in value added terms than a non selective school whose results are lower overall but far better than their intake implies they should achieve.
In addition, there will be an element of parental and pupil driven results at a selective school. Nonsuch's reports states:
Students and parents who spoke or wrote to inspectors suggested that, for some, outstanding examination success is helped by additional tutoring outside school and parental academic guidance that complements good classroom learning.
This doesn't mean teaching is poor - the report states that it is good and only one area of concern regarding differentiation is really highlighted. It probably just means that the school attracts a lot of pupils whose parents would probably tutor and push for higher results no matter which school they attended and no matter how hard the school pushed its pupils.