Any setting >= 768k automatically puts your server in the global zone (@), and if you change it lower then it is automatically removed (though you will continue to get queries for quite some time until clients reboot and refresh their NTP server list).
You could be receiving queries from china because like you said they are just scanning IPs, or it could be misconfigured netblock(s) in the Maxmind GeoIP database, or they could be using the europe pool name.
Having 3 tiers (Global, Continent, Country) gives a good level of flexibility for clients to choose how broad a pool they want to query from. It also allows for the registration process to be fully automated. Also, just because two countries are adjacent to each other doesn’t necessarily mean they have good internet connectivity between.
Having the continent zones makes adding a server to multiple country zones within the same continent redundant. It also dilutes the level of precision for end-users that want a specific country, and end up with a server that really isn’t located where it says it is.
IIRC, simply using [0-3].pool.ntp.org will automatically use geolocation and return servers within your determined country.