Because of Hermione’s vigilance, Harry gets a glimpse of how his nemesis behaves when he thinks nobody’s looking. Snape is not acting here. He is careful even with the unconscious Sirius Black, in contrast to Sirius bumping Snape’s head intentionally when Snape was unconscious. (HP/PoA, 378) It may be that the private Snape is a better man than the public persona Harry usually sees.
