I don't believe there is a rule about specificity or individuality in terms of possessive nouns. You can be a "horse's ass" or a "tavern's patrons" or a "year's worth of whiskey." A year is a pretty non-specific, immaterial idea to possess something, but nevertheless, that is the usage. It's a "world's fair" even though it's created and hosted by one country (with other countries' participation).
Many uses of possessive apostrophe are arguable, or simply traditional, and not always logical. In the examples given in the question, there is a difference in meaning or connotation.
The health organization is concerned with the world's health, or with health around the world, but it is not owned by or possessed by the world. And, it does count for something that "World Health..." is easier to say than "World's Health...."
Put another way, the "League of Nations" could also have been the "Nations League" and the "United Nations" (simple plural), or the "Nations' League" (possession of the nations).
"America's Credit Union" is trying to suggest that it "belongs to" America; although of course, it doesn't.