Mech is right, ATAR has nothing to do with quality, it's the demand of a course.
I don't understand why you single out one particular person with an 83 as their ATAR, do you know nothing about statistics? That person is a SEAS applicant and in some ways, you can consider them an outlier, which means that they shouldn't even be included in statistical calculations.
I hope the same as Mech, really, had you got a 99.00+ ATAR, your attitude might be somewhat forgiveable and understandable as you have a choice of other degrees with stronger cohorts (such as BBioMed), but with a 91.35, you'll probably be in the bottom 25% or so of UoM BSc students, so you can't exactly say that you're better than the majority of the cohort, so it's not as if you're in the top few, really don't look down on other people.