I think it has more to do with stallions registered with the Dutch Warmblood Standards, there was a post on the subject earlier this week with more information
I was just looking at horses for sale on a website and one owner had put that it was a belgian wb but registered with the KWPN. But another said it was dutch so i got a little confused. As i said before I don't really know anything about the different warmbloods so thats why I posted.