Well, you know that PER numbers care more about offense than the two ends, it's not surprising that Kawhi has his first season in top 5 averaging +20 points...
Also, having Pop as coach, makes Kawhi doesn't play too many minutes or take too many shots.
David Robinson in his best season took 20.7 FGA per game and played 40.5 mpg.
Kawhi Leonard in his best season is taking 15 FGA per game and playing 34 mpg
Of course "not the an apples-to-apples comparison" because Kawhi will never have the chance to play DRob minutes or take that many shots.