
Because Hips and elbows are not wholly genetic, opinion, knowledge and experience come into play.
Elbows are far more heritable, from articles it would seem twice as heritable as hips.
Both are genetically bilateral, so differences re sides should be small, larger differences point to non genetic damage effects.
In hips both sides are always given in results.
As so few of my breed are elbow scored in my and all UK results are 0, not sure if published results show both sides + total, or just 0 - 3 without differentiating, though certainly Scandinavian KC's show full result, which is why I know there have been a tiny proportion of 0/1 scores in the breed.
Breeding is a balancing act taking into account many aspects.
With DNA testing for example one may often find ones dogs Clear for some and Carrier for others. Choice of partner to compensate with Carrier to clear avoids producing affected offspring.
Hips and Elbows are less clear cut, and relatives, and ancestors scores will help determine a good or bad pattern.
In a few breeds ebv's have been developed (using that information) making decisions a little simpler.
The there is nature/luck/Shit happens.
Also so much we can't test for/see and rely on keeping records through the generations of the less tangible selection criteria.