I believe the results are the product of Law student surveys, so I would make an educated assumption that it refers to work in the legal field as opposed to general work.That data can easily be attributed to the fact that the cohorts of UNSW and USyd are probably stronger than those from other unis. Also, what "work" do these people find? Is it work as a lawyer, or just general work?
As to the cohorts: stronger cohorts would likely mean better employment prospects. If you went just off that, you would logically pick the uni with the higher ATAR cut-off (I believe that is ANU rather than UOW?)