you CAN get 100 for you HSC ie. the thing that the board of studies gives you
UAC then comes along and goes 'right let's scale these bitches'
and then all your marks get pushed up and down, you don't get to see what they get scaled to yourself, but basically
if you get 100 in art, it won't contribute 100% to your UAI
if you play around with SAM you will see the pie chart that comes up on the right after you put in marks and you see the percentages that come up, that's the amount that subject contributed to the calculation of your UAI so it'll make more sense when you look at it like that.
just work on getting the highest mark possible in subjects like art and that and don't worry too much about how much it's going to scale you down, cos it won't unless you fuck it up