I agree. If it were only about your performance in the aptitude tests then I don't see why they'd even have the group ex and interviews. In fact, I'd go as far as to say that the interview is the make or break for people who make it that far. In other words, if they don't like you in the interview then it doesn't matter what you did in any of the other tests; you're probably not going to make the cut. Whereas if they think your interview was excellent but you didn't perform as well as the five engineering graduates you're competing against in the maths/physics tests, you're probably still in with a very good shout.
Only in the interview can you really directly sell yourself to the interviewers, and I'm pretty sure they would be willing to overlook a few rust-induced elementary arithmetic errors if you are otherwise exactly what they're looking for. But let's face it, the maths and physics tests aren't difficult for anyone who has the entry qualifications to apply; and given candidates have had time to prepare there's very little excuse for rustiness at the basics, irrespective of whether you are Hawking or a just-out-of-school kid.
