The program treats all postal codes as strings of characters, without caring about numbers or letters. There may very well be some interesting patterns to be found by ignoring the large scale, and just looking at the lowest levels. As it is though, the program treats all countries exactly the same.

As for the Netherlands: what might be fooling us here is that the country is of course much smaller than, say, Spain, so it tends to look sparser at a comparable size. But it is very well possible that the data is simply incomplete.