That’s just the law of small countries (just made up that term). It’s easier for smaller entities to be an outlier. You’ll find about the same positive as negative ones, though.

If you were to split a large country such as the USA or China into parts with a third of a million people, chances are that, for any of these, there would be a parts that beats Iceland.

There may be exceptions such as the highest usage of hydro-power usage, as they have a rather unique geology. Vatican City won’t be beat on popes per km², either (it has just over two)