text: There is no postal code defined for Sambreville, so Nominatim "has to improvise". See http://overpass-turbo.eu/s/11bu for postal code boundaries around here.
  I think it just takes the closes postal code, https://www.openstreetmap.org/relation/4240188
  If Sambreville=5060 (no other postal codes in Sambreville, no other municipalities using 5060), then you can simply copy the existing municipal boundaries and add them as a postal_code boundary