The Dark Lord, for instance, almost always knows when somebody is lying to him. Only those skilled at Occlumency are able to shut down those feelings and memories that contradict the lie, and so utter falsehoods in his presence without detection.”
The fact that Lupin told Harry the day before this that Snape is skilled in Occlumency was a foreshadowing of Snape's true allegiance. He can deceive the Dark Lord because he is a skilled Occlumens.
