During his Celtics days, he was hyped up as some defensive monster that had all these "intangibles". There were a lot of myths built around him that added to his value. When the Celtics lost to the Lakers in the finals, it was because Perk couldn't play. When he was traded, and the Celtics lost in the playoffs, it was because they got rid of Perk. He was just well protected in the Celtics system, and they covered up his weaknesses.
All of this crap made him a lot more appealing than what he really was, and the Thunder traded for him thinking there were getting just what they needed. They then signed him to an overpaid contract, before even checking to see if he was worth it.