I'm currently working for a UN-agency (as a contractor) and the salaries for staff are fixed based upon your job level and number of years of service, so basically everyone knows what everyone else gets:
http://icsc.un.org/resources/sad/ss/sal201201.pdf
A senior developer is usually P-3 or P-4. It may seem on the low side compared to market rates if you in New York or Geneva - but bear in mind they don't need to pay taxes and get quite a lot of other benefits.
Edit:
This site has more details on the salaries, benefits and theory behind it: