In the previous season, Klay became the third, sometimes fourth scoring option on his team, as opposed to being the first or second shooter on the Warriors as he was in the past. This however didn't influence his stats, which still were exceptional. A streaky scorer, Thompson doesn't need to have the ball in his hands to score 20+ points a night.
His rating is lower exactly because the decrease in usage wasn't reflected in the decrease in production. In other words, Klay Thompson is a great, but one-dimensional player, so 90 for someone who can score 60 points in less than 30 minutes seems a fair assessment.