Neil Hunt, Netflix's Chief Product Officer, explains why the streaming selections vary from country to country on Quora:
Each country notionally has a different catalog, although there is substantial overlap. Licensing is generally for a region (e.g. Latin America, or Nordics), so countries within the region have very similar (but not identical) catalogs.
Sometimes a shared title is the same media in each country; sometimes the title has been edited (e.g. for ratings compliance, or just to add or removed a channel bumper), in which case it's actually a different media file. In general, the ID spaces are separate by region.
I'm sure that trying to secure international rights to some titles is probably a nightmare, but as Netflix grows it might get easier.