Answered by: James L. Cook, DVM, PhD, DACVS, DACVSMR:
There are numerous in-depth articles on this question, but in short, the reason this can happen is that HD has a polygenic mode of inheritance (many genes involved) and is a multifactorial developmental problem (the hips are normal at birth, but develop abnormally) based on genetic, nutritional, and activity variables. So, like combining two hands of cards, the “right” breeding combination can produce a good hand (no HD) even if each individual hand was not good and vice versa, especially if the nutritional and activity related factors are done well or poorly, respectively.
