In large part because Breivik completely altered his behavior between examinations; he is smart and well educated enough to know what they seek and how to present evidence for his preferred result. However, he spent a decade planning his killing spree without ever showing any sign of mental instability. He was also careful to avoid showing any sign of criminal behavior; he does not even have a speeding ticket on his record, because he did not want to draw police attention.

So, for ten long years, he did not, even though he spent that time meticulously planning his massacres. He joined a gun club so he had all legal access to guns possible under Norwegian law, but did not immediately rush out and buy guns: He waited YEARS to buy his first guns, just to avoid suspicion. His cop uniform was so convincing an off-duty cop on Utøya walked up to him believing him a colleague responding to the Oslo bombing, only to be shot in the head. He started a farming company, with all the legal paperwork in order, just so purchasing massive amounts of fertilizer would seem innocuous. He gives every appearance of being a perfectly rational cold blooded killer (except when telling one court psychiatrist he is suicidal before telling another he has never been suicidal, but does that suggest a rational or irrational mind...?) He does not even have the classic sociopathic inability to distinguish between right and wrong: He admitted from the outset his murders were wrong and horrified him as well, but claimed them "necessary."

From what I have seen, the evidence for/against his sanity is inconclusive, but "inconclusive" was not among the rulings available to the court.