According to Synergy Sports, Joakim shouldn't even be on this list.
Tim Duncan is warranted. He ranked 57th defensively.
Perk ranked 80th.
Joakim ranked 240th defensively overall. 
Boozer ranked 280th.
Paul George ranked 258th.
That's raw points per possession scored against data that's not filtered. Can't really use that as a decider. A team with a philosophy to switch on every pick and roll can mess the stats up for a guard with terrible defensive bigs, as an example. For Synergy, the safe bets are to go with isolation for guards/sf's and postups for Bigs. Tweeners like Josh Smith are kind of lost in that, though.
Perk allowed .78 PPP in postups.
KG allowed .71 PPP in postups.
Just thinking about how different they go about post defense as well. KG is much more aggressive at denying the ball to post players. Still rates up very well even after the catch too.