Whilst sympathising with the "why has this happened to my DC" I do struggle with Oxbridge candidates being unfairly treated when I stop and think as to what the logic might be.
About 75% of Oxbridge applications are unsuccessful, in most of the STEM subjects (for Cambridge at least), the percentage unsuccessful is even higher than that. What logic would therefore exist to rule out such candidates on the basis that they may have applied to Oxbridge when the majority of them won't actually be going to Oxbridge?
Given the numbers applying do admissions tutors / staff really have the time to ponder on whether a candidate may have applied to Oxbridge, and the ready availability of a crystal ball to tell which will get an Oxbridge offer and quite possibly therefore not make their particular institution a firm choice and to be dismissed out of hand accordingly? Whisper it, but some Oxbridge candidates, perish the thought, might actually have another Uni as their ideal first choice.
TSR? well on the lies, damned lies and statistics continuum it could well be some distance on from statistics in this instance.
We know that more offers than anticipated places are made. Let's take Durham, a regular choice for conspiracy theories, although, on an overall level, not ranked 7th by any of the three league tables (in alphabetical order it is 5th, 8th and 6th in the current tables) so possibly not the culprit in this instance :). A range of very popular courses and roughly 3-4 offers made for every potential place available, although a few outliers - I was a bit surprised that in the last cycle for which Durham have published data 97% of applicants for Archeology received an offer, and 91% for Engineering, although there may well be very good reasons for this . It is not as if such Unis are making limited numbers of initial offers, but there is a good sized pool of well qualified candidates, and some will therefore unfortunately not make the cut for whatever reason.
Whilst anecdote pains me I do have to wonder whether over processed (by schools) and formulaic PSs do trip up some candidates, who might otherwise seem to tick all the academic boxes. anecdote alert Certainly having sat in on a talk at one elite end open day the director of admissions named a fairly well known boarding school (not in the Eton, Winchester level of public recognition) and stated bluntly that it appeared that the school told all DCs wanting to do subject X to apply to this university and include a,b.c.d,e,f,g.h ... in their very formulaic PSs. The inference being that many then ended up in the reject, rather than the offer, pile.
Whilst it might be annoying not knowing why an application was unsuccessful, having an offer in the bag from somewhere as well regarded as Warwick is an outcome that the OP's DD is still hopefully very pleased with.