Why did Batman say he was going to rescue Rachel but then he rescued Harvey?

In the prison when Batman was interrogating the Joker, he asked where everyone was, and the Joker being the trickster that he is mismatched the whereabouts so that he would find Harvey, not Rachel because he had a plan for Harvey to get into his dark side, as he had "chosen" him. He "chose" Harvey because he was the "best" of people among them, and he wanted to show that even the best had an evil inside them.