If 280 kids apply to SAS then yes about half will get offers. However, the cohorts applying to and ultimately accepting places at SAS and Aldenham are likely to be very different because the schools are very different. There will be quite a bit of self-selection at the application stage.
As SAS is known to be an academically high-achieving and demanding school, you won’t get that many academically below-average/struggling children applying. Any children coming from prep schools are likely to be steered towards other schools. To the extent less-able kids do apply they will largely be weeded out during the assessment.
In contrast Aldenham is known to be much less academically selective and to have a different offering. Many of the children there wouldn’t be at the academic level required by SAS (which is of course fine, kids have different strengths and needs).
However, I’m almost certain that a number of quite academic kids do apply to Aldenham, but use it as a back-up if they don’t manage to get in to schools like SAS/Habs etc. Most of these kids will get and will accept offers to the more academic schools. What it means is applicant numbers to Aldenham are boosted by candidates for whom it’s not their first (or second or third) choice. Aldenham may have to make a higher number of offers than SAS in order to fill their places.
As an aside, I’ve never entirely understood how oversubscribed but non-selective schools decide what offers to make!
Given falling birth rates and VAT, standards at academic schools as a whole are likely to fall a bit, it’s just one of those things. The alternative is for schools to try to maintain standards and inevitably contract which would be a risky strategy.