Under the headline It's a record: 23 HSC students hit 100 the SMH on the weekend confused the UAI with a score.
The UAI is a rank - the candidates are all ordered (actually they apply a notional rank to the population old enough to sit the HSC who don't) and given their rank in the UAI. There is basically the same number on each index point. If the total population increases (as it has) the total awarded a UAI of 100 will increase.
These students did not get "perfect" scores, they got the "best" scores - which is an equally great achievement. And they are a group of students who surely know the difference between a rank and a score.