I'd be very surprised if they are teaching those working very low or below keystage in those groups. Those children require a more personalised provision and often access the curriculum differently.
They might just not have advertised that to parents for various reasons.
But once you remove the very top and very bottom children, the range of the rest isn't unmanageably great.
Setting too rigidly can also be damaging because the lower middle children (for example) don't have more able children to bounce ideas of and the range of discussion is quite limited. Eg in English, they are only exposed to the thematic analysis or the vocabulary that they and their peers are capable of generating. Sometimes they need the inspiration. We're quite capable of supporting a range of lower to higher middles because the range isn't generally that great.
For example, there are children in my maths group (top) who are streets ahead and others who it was a fine line whether they were in my group or the one below. You can hear them explaining concepts to each other and rationalising thought processes - even in the top set there will be a couple who stand out as being well ahead of the others and a couple you're trying out to see how they get on. In the middle groups, this is more beneficial because, as I said earlier, the higher middles benefit from explaining their thinking and the lower middles benefit from hearing it from a peer and refining their own thinking.
Not only that but where do you draw the line? There isn't a simple and clear distinction between the majority of children. All of these groups/sets are 'best fit' because they're children and the decision is based on a number of factors - ie not just how they perform in a test. Eg confidence, personal pace, application/transference of skills, clarity of thinking.
We rank the children based on assessment scores and then have a discussion based on several factors before setting.
It would be naive of parents to think we just look at test scores and put the top 10 in one set, the next 10 in another, the next 10 in another and so on or whatever numbers are involved. Because it just doesn't work like that.
No one wants any child to be held back or struggle.