I don't know the answer to that and it is indeed a good question.
BUT...generally speaking at the onset of arthritis pain is worse on physical activity both in the knee and hip...depending upon which joint is affected.
As the arthritis becomes more severe, then the following pain characteristic appear:
Arthritis of the knee....the pain is much worse at rest than during activity.
Arthritis of the hip.......the pain is much worse during activity than rest.
Generally speaking........and of course ABers will post their opposite views.
Nobody really knows the reason for this opposite reaction in these 2 large joints.