My DCs' school also split the year group for KS4. It was to facilitate triple science, which was taught in the same number of lessons as double, plus one extra a week, dropping one English or maths (alternately). Obviously this would require higher ability students, both for the pace of the lessons and to cope with the dropping of a core lesson a week.
The split did not mean all the double science group took foundation maths tho, for example. Nor science. It also didn't apply for option subjects as they were set all at the same time so could be quite mixed ability.
To answer your question above – (and I am a teacher) my experience is that children who are brilliant at maths and rubbish at English are pretty unusual. DS2 was and is a scientist but he was more than capable at English. He wasn't a massive fan and didn't get an especially high grade, but he is a smart lad so needed to be in triple stream (if I can call it that – it wasn't termed thus by school). Equally DD – loved English so so much, not a big fan of maths, did enough to get a solid 6. Again, no reason for her to be in a lower maths class as she was able.
Yes that is just two YP but I have taught thousands of students and honestly, I could count on the fingers of one hand those who would get a 9 in English but would struggle to get a grade 2/3 on the F maths paper. Or vice versa. Yes, as noted, maybe the maths fans won't try that hard at English or history but they could. And yes I know spiky profiles do exist, but they are honestly few. I think in the example you give that student at my DCs' school would have been in triple, perhaps with extra English support. School perforce has to work for the majority. Perhaps that's not a great thing and that's a different debate IMO. But to hold DS2 and his peers back in science, maths, English to the pace of the double groups – or to speed those kids up to triple pace – would not have worked.