The ZW number really needs to be looked at as part of the whole family - the littermates, half siblings, and their progeny too. If a female is koered in Austria or the Netherlands or Belgium and only 1 or 2 of her pups get into the system and are FN, and then she has a dozen pups whose hips go A or OFA Good and Excellent...her number can be really skewed! My own female has produced 2 FNs and a Fair from one male, and from 2 other males, OFA Goods and Excellent (both adult and prelim films). Unfortunately, only the pups from the first male in Europe are done there, so the Goods and Excellent are not affecting her ZW number which would go down dramatically otherwise.
As Trish says, some dogs are known to be improvers (Aly Vordersteinwald for example) and other dogs are not improvers, and need to be bred to females with good hips and lines. Some dogs are "known" to be poor hip producers, but are producing some other trait well, so are used, but need to used wisely.
Lee