It means you can tip the scales a little one way or the other. Some CHD-affected dogs will show little or no signs of lameness while others will be so severely crippled by the disease that only expensive orthopedic surgery can relieve the condition. Dysplastic pups can be produced from several generations of clear individuals because the disease is so complex genetically. Looking at hip clearances on the ancestors isn't enough.
Accurate history can be hard to come by.
Breeders are sometimes less than forthcoming. A breeder could easily remain unaware of offspring that actually were affected. Not every dog with CHD requires major medical intervention, so owners may not become aware their dog is affected. No CHD-affected dog should be used for breeding. Any dog, no matter what its rating, which produces multiple affected offspring, especially from different and unrelated mates, should no longer be bred.