based on a number of things. Basicly, there is a suply/demand factor going on. The more people that want to do the course, the higher the enter will go. This is because a uni can only offer a certain amount of places in a course. If, for example a law course offers 50 places and 100 people want to do it, they then get to chose the top 50 people, and this pushes up the cut off (note that this means the cut off is created retrospectively- past years are just a guide). If another law course also offers 50 places and only 40 want to do the course, then (assuming they don't have a minimum enter) someone who gets a very low score would get into the course because there is a spot left for them.
So, the cut offs are pushed up by what year 12 students percieve are the better courses. This means that the most trivial things can come into the picture (some people chose one uni over another because of the apearance of the uni grounds), but we can assume that most kids will pick the uni which they percieve will give them better job oportunities.