This list looks to go by a unknown filtering criteria for cities. It includes multiple cities in some countries, and limit it to just the largest city for other countries.

For example, the work conditions in the various cities in Norway are probably more or less the same, and if anything, the criteria by Air Quality, and cost of living should place Oslo further down the list than others. And one can wonder why Bergen, Oslo, Trondheim, Tromsø, Stavanger, Bodø, etc. are not on the list.

The answer to this is sort of implied: This list only includes the cities it includes. The important takeaway is that this isn't a list over the top N cities to work in, but rather how a selection of cities rank in this regard.