You're correct about the definition of NUCs. They were introduced in order to have a common standard for international tickets. (By definition, a NUC is exactly US$1.00.) The "NUC" was introduced by the world's airlines because it is more "politically correct" (the airlines -- mostly non-US ones) didn't like the idea of a Capetown to Hong Kong fare being priced in US$ (instead, say hello to the NUC.)

I believe the NUC is based on that day's "bankers rate" for US$.