For there to be 38 veg*n restaurants per million of Icelanders, there has to be about 15 veg*n restaurants in the country (since there are only ~384,000 Icelanders). That's still quite impressive.
A couple of the reasons why Norway's numbers are low: The population lives competitively more distributed across the land area, so they have to travel longer distances to visit restaurants. Also, wages are higher, which makes running restaurants more difficult. So it's about the restaurant business as a whole, not specifically about veg*n ones.