The AON tests probably cost money per candidate for BA to put people through given how much more in depth they are than Stage 1 and the fact they're involving the schools. Far easier to give a motivational answer which takes under a minute to assess.
It's only fair that the tests are very mixed at this stage. They have been clear from the outset that they don't just want cognitive ability and the short writing exercise given is an easy way to assess that. Read the question over and over again and ask yourself if your response directly answered the question. Best of luck for next time.