Forgot one: Use of privacy-minded anycast DNS resolver services such as Quad9 and Cloudflare.
With those, the pool is not able to detect the location of the client directly (because the DNS resolver hides relevant information from the pool’s DNS servers). Rather, the pool then takes the location of the DNS server as the zone for which to provide NTP servers to the requesting client. And if the specific DNS server instance forwarding the request to the pool’s authoritative nameservers happens to be in another country, that is what the pool will assume as the client’s zone.
There was a nice presentation at one of the last RIPE meetings that nicely illustrates the “watersheds” for anycast DNS resolver services. The figures are rather coarse, but I think one can roughly see which geographic areas are served by which anycast DNS instances, and that in various cases, regions are served by instances in other geographies. E.g., because the region/country doesn’t host an anycast instance. Or just because the routing in the Internet, at that time, happened to send requests from one place to an anycast instance in another country, even though the origin country did have its own anycast instance.