This is because there is no exact match between neighbourhoods, postal codes, towns and cities. Neighborhoods and residences are formed by municipalities. Postal code areas are defined by PostNL.
In addition, residential areas are demarcated on the built-up area, neighborhoods run outside it. Also, there is no 1 on 1 match between a postcode and a neighbourhood. If a neighborhood falls under a zip code area, that is the zip code area that occurs most often in that area.