If PER was an accurate measure of a players actual performance and If said players opponent was accurately recorded then yes it would be a be a good statistic! The truth is that whatever site you are using is most certainly not recording who a player is guarding! They are making assumptions and marking who they assume was the center and power forward and doing the same for the other team! They then assume one center is guarding the other teams center and so forth! In the case of K.O. and Sully this results in Sully being marked as the PF and K.O. the center when in truth K.O. is more often then not this year actually defending the PF position when on the floor with Sully! The same is true for Rondo and Avery, Rondo is being marked as defending the other teams PG when in fact he is not more often then he is when on the floor with Avery! This results in a very muddy mess and is useless!
Look, I personally am a huge critic of PER - it is a lousy stat for comparing different players for a host of reasons that I have ranted about frequently.
But the way it is used by 82games.com to record the opposition performance and the positional 'Net PER' is actually pretty reasonable because it isolates the role context.
The position assignments are certainly subject to error, but in the aggregate are probably pretty close. Especially on the defensive end.
Absolute precision isn't really necessary when the differences illustrated are this big. Further, the Net PER results used here are completely consistent with what has been measured on the scoreboard, as data posted in this thread illustrate.
It is certainly true that 15 games is ultimately indeed a small sample and I personally wouldn't read too much into the numbers from just that.
But the numbers are a record of what has happened so far and even if you don't want to accept them as proof that Sully is a better defender than Olynyk, you still have to reconcile that they are completely at odds with the OP assertion of the contrary.
And the numbers from last year's much larger sample also don't support the OP.