Nope you dun have to...they just publish the uai's higher than required..for most non popular subjects anyways. Like UNSW says it requires 90.5 UAI to get in arch for 06...Exactly my UAI, and I asked him what are the chances of it going up for 07, and he replied that the REAL cut-off for 05 and 06 was like 85...So just always place what you want to do. I asked UTS and they said round the same thing. People with 83/4 get into 89 UAI subs (i was asking for architecture but then she started to generalise). The UAC book is only a GUIDE
But of course i doubt it will be true for Law and Med..
They make the UAI high to show off that they are a good uni. Remember that a Uni's survival, income and fame are depended on future students, and the more the better. They try to make you put first pref, thats all.