
As genetically hip dysplasia is a bilateral condition the real geneitc influence shoudl mean a fairly even score. when ypou get an uneven score this is where it is likely that environmental factors have had their effect.
With a one sided score I would always take the true genetics as being something similar for the worse side, so I would expect the high side to really have meant to be no more than a couple of points worse than the better side, especially with siblings to compare to.