So 6 means you can get max possible, but you only get 5-6 in the north. 1-2 noob river and a bit north of it. 3-4 middle to north desert. I forget what they said for swungle but I’d guess 3-4. So put a base in the middle zone of purges not just far north.
This info is of course based on how it is supposed to work. Apparently your mileage might vary.
If they set max to 4, then even in the north you’d only get a max 4 difficulty purge.