A degree has x amount of places in it. The cut-off represents the ATAR of the last successful applicant which filled up the degree. If it isn't filled up, the listed cut-off just represents the lowest ATAR of all the successful applicants.
I'll try to explain: the cut-off of most courses (except those that are fixed such as Medicine, Law and Engineering) tend to vary based on supply and demand through the rounds but it can also reflect the ATAR of the last student to be accepted into the course (during Main Round most of the time).