I don't think Mathematics deserves '76' ATAR. my questions is..what is the most significant difference in this course comparing to other courses under science department at Melb or Monash?
1) Are you saying that an ATAR of 76 is too high or too low? Because if you're saying that it is too high, both Science at Monash and UoM have higher clearly-ins than 76. Secondly, the ATAR required is not representative of how hard the course is, or what level of intelligence it requires, it's a measure of how competitive it is to get into a particular course. Thus, in many ways, students decide what the clearly-in ATAR for a course is.
2) There won't be any significant differences, whatever branch of mathematics you decide to go into will be based on your subject choices. I'm not sure about other universities, but at Monash, at the undergraduate level, there isn't significant breadth in subject choices anyway, all maths majors will probably spend the first two years doing similar subjects, part of their third year will also be similar.