I imagine that "UN expert meeting" is to be parsed as "[UN [expert meeting]]". As in, a meeting of experts that was hosted by the UN. It's not a meeting of people who are "UN experts" (i.e. experts in the UN).
Discussion
The aspects of being from the United Nations, and an "expert" instead of peer reviewed scientists, are what had me shaking my head. But now thay you mention it, I think that probably makes them an un-expert too. 😉