From the methodology section in the linked Nextcloud blog post, this is an interesting thing to note:

The DSI score uses a number between 0 and 100 to show the number of deployments per 100,000 citizens relative to other countries in the Index. [...] As small and medium businesses as well as private users running servers far outnumber larger organizations like government or big enterprises, the index says more about the choices of individuals and small companies than what government or large corporations do.

This score measures where the services are deployed rather than where their users reside. For example, users from UK with services deployed in NL would shift some point towards the NL score. I don't know if this makes a significant impact in the overall rankings, but NL is a popular hosting location for VPS due to a good combination of price, speed, and lack of censorship, which may help explain its good score.