Why does Draco Malfoy act like he doesn't know if it's Harry Potter?

In The Deathly Hallows, in Malfoy Manor, why does Draco act like he can't be sure that it's Harry, Ron & Hermione? I thought at first that he had a change of heart.. but then later at Hogwarts he tries to capture them to turn them over to Voldemort. I've never quite understood.. Any thoughts?