> They make offers expecting that the vast majority of them will be met, rather than over-offering and expecting A-level results to prune the numbers significantly.
Notable exception to this would be Cambridge maths, who over-offer significantly for Mathematics (last year: 541 offers, 253 acceptances). Quite a lot of people miss their STEP offer so colleges can choose on results day who to take based on STEP, UMS, interview, etc. Though STEP unlike A-Levels was sat this year, so they're still able to do that selection.
(Oxford, along with Cambridge but unlike many universities, has the results of interviews and, for many subjects, admissions tests to assess to whom to offer a place. They make offers expecting that the vast majority of them will be met, rather than over-offering and expecting A-level results to prune the numbers significantly. In previous incarnations of the admissions system, candidates who passed the entrance exam and interview routinely got an offer of two Es, i.e. a formality. So, providing places to all offer-holders would not seem an unreasonable course of action in the circumstances.)