They explain it by saying it gives them time to go above and beyond the syllabus.
I'm sorry, but with extensive experience of both my own children, and close friends' children, in various super-selective settings, that's just bollocks. Firstly, you have to ask why: unless all the pupils are getting A*, why would you spend time on things not on the syllabus when you could be doing work to improve their grades? Secondly, you have again to ask why: given such schools teach up to twelve (and in some cases more) GCSEs, and pupils then drop down to three or four A Levels, what is the benefit of all this extra depth? Thirdly, what evidence is there that this claimed additional work helps the pupils and A Level or later? And finally, and most damningly, what evidence is there that it actually happens: if you have the subject knowledge, why not ask the school to show you a lesson plan for some work that goes above and beyond the syllabus? Because I can virtually guarantee you that it's a shadow,
I'm afraid it's one of those self-serving myths, rather like the justifications made for teaching iGCSE even though it's easier and now the favourite of failing comps or switching to Pre-U even though most schools have rejected it out of hand and universities treat it like plutonium.
A certain sort of school tries to claim, and a certain sort of parent is willing to accept, that GCSEs are in some way debased coinage, and are so easy that anyone could get a A, so we as a school have decided to switch to an alternative which offers more rigour, depth and has both a wetter nose and a glossy coat, which universities can't get enough of. OK, headmaster, presumably you made this decision in the aftermath of that year in which everyone did, in fact, get A in every subject? Ah, no. That's odd, headmaster, because if everyone can get an A* in every subject owing to how easy the exams are, how come your pupils don't? Ah, says the headmaster, that's because we're doing things in more depth and using more rigorous exams.
Parents and the school can then explain away why a cohort from the 2% of the ability range, taught usually in smaller classes, without any of the issues of dealing with FSM, EAL and so on, could still only manage to get about 10% of the cohort, if that, through to all A* at GCSE. And they do it by taking other exams that allegedly have more rigour, although there is no evidence beyond the marketing material of the awarding body of this, and/or claiming that they have their KS4 teachers focused on matters far more intellectual than mere GCSE results.
It's just a smokescreen.