Other LE posters said it was a technique, a step by step process to get him to confess to all the murders. They felt he had killed all 3 but knew Shanann had not left that house by her own volition based on how the house was locked from the inside, her purse, phone, meds all still there. They knew he would fail the questions about her, they could use that to put pressure on him, open the dialogue about the girls. I see it as a judgement call as how to go step by step to get him to tell the truth about all 3.