Something else is going on here besides weighing less being less hard on the knee joints because the ailing knees of those who lost the same amount of weight through exercise without dietary changes did not benefit. Perhaps whatever dietary changes people made while trying to lose weight had an effect of reducing inflammation, insulin resistance or improvement in some other factor? Even if they followed low fat calorie restricted diet plans, most people on those plans will cut out junk foods, soda, excess sugar, etc. so some improvement in health would be logically expected from that.