I think that adaptive tests are particularly difficult to predict and can give DC the impression that they have bombed when in fact they have done rather well - as the test get trickier (because they have done very well in the initial section) the DC might get the impression that they are 'bombing' it when it is only normal that they take more time or become more tentative in later questions. (From the conversation at the time, that may well have been the case for your DD - the 'bombing' was actually a good day where she was reaching higher levels of difficulty that, in turn, give more points).
All in all, as you say, very difficult to predict and depending on the type of exam. And, of course, the day... for instance, both KGS and LU have rather similar exams, we know of kids getting offers for LU and not from KGS (and even more the other way round)