nlondondad: "People had an offer but declined it due to a change in circumstances, so it is re offered."
Exactly. A "change in circumstances" such as moving house, going private or deciding to home educate. Some of that will be natural turnover, but I strongly suspect some of it will be forced by the reduction in surplus school places.
I don't know Islington, but in my community the second effect is very tangible. Aspirational families (from a broad range of class backgrounds, but upwardly mobile) make significant life choices based on whether or not they can get a reasonable (i.e. Ofsted Good+, and logistically convenient) school place. The result is a very unstable community. (And yes, people move for other reasons too - house prices and employment, but the supply-and-demand of school places has a significant influence on their decision-making).
The raw numbers from Islington tell us little about whether there are similar effects at play, but that is what I meant here when I said that the social consequences of the national policy to decrease school place surpluses weren't being audited quite as closely as the raw numbers.
The numbers of families turning down state-school offers needs to be monitored, and if they are rising then the reasons for that need to be looked at, because it wouldn't be a welcome trend. (Perhaps welcome for bean counters, but not welcome for people who care about state education).
And even if the effect is only detectable in a small number of relatively affluent LAs at the moment, it is likely to spread as surpluses reduce futher in other areas.
Like the NHS, the more people you have going private, the less those with influence care about quality in the state sector. We don't want to encourage a "well you get what you pay for" attitude or a "those that can afford it should stop complaining and go private" attitude in education. That would severely let down those that can't afford to make contingency plans.