The ambition of the UN Job List is to cover the UN system vacancies as much as possible. To get an overview over the organisations which are part of the UN system, have a look at the UN organizational chart.
The UN Job List features vacancies you find on the official job websites of the different UN organisations as far as they are advertised centrally. To get an overview over the different organisations as they appear on the UN Job List have a look at this list: http://unjoblist.org/lists/Organisation/
The reason for this kind of focus is simple: It’s a matter of choice and simplicity. Choice because I just happen to believe in the UN Charta and simplicity because I’m able to build more effective filter and search tools since I can focus on UN contract types and contract levels, only.
Please note that the UN Job List’s focus on the UN does not mean that there are no other valuable job vacancies out there in fact there are many other valuable and really great NGO and other jobs out there, too.