Teaching styles might be different as well and there is more scope for teachers to deviate and/or go beyond the curriculum. There may be less adherence to the (rubbish in my opinion) structured lesson taught in training colleges. For an excellent teacher this is liberating , for an Newly Qualified Teacher it can be daunting. The teaching is not better, but it will be that the students will cover the material faster and will therefore have time to explore around the prescribed subject.
I'm curious about how qualified you are to make these 'observations' about teaching. There is scope in any
school to enhance the curriculum and I find some sort of 'structure' to a lesson enhances the learning. I have been teaching for many years and have continued to develop and research teaching and learning - some of the best practice I have seen has been in non-selective schools.
This shocked me a little too - total sweeping generalisations so I would be very curious to know on what they are based. I have only ever (by choice) taught in the state sector and (also by choice) have tended to work with children who are definitely not what you call 'top set material'. My classes have variously had lessons in Swedish and Russian; in natural history and ornithology; and in cookery and current affairs. None of which was on the curriculum.
And fwiw I always did OK OFSTED wise too, and on my dreaded 'impact reports' as the kids managed to pass their exams as well.
For the OP - never discount 'snob value' when looking at some of these schools - there is a hierarchy which may well be self-fulfilling. A bright and motivated child from a supportive home will tend to attain well when in an environment in which she or he is happy, regardless of labels put onto the school.