I think that the common scale is mostly going to be used to compare the difficulty between adv and standard math. I think that in past years there was a problem with people choosing standard over advanced because it's easy to do well and when you do well scaling doesn't really matter. This kinda removes the incentive of doing advanced unless you're doing extension leading to smarter students choosing an easier subject despite being capable of advanced. This also discourages hard work as a smart student can choose general, do almost nothing and still do well + receive a good HSC mark. The common scale should give averages for the strengths or each cohort, e.g. adv math students' avg for common questions is 90% and gen math students' avg is 60%. This would show that adv math is significantly harder than general and scale adv significantly higher relative to general (although the numbers prob won't be as drastic as my example).