Draco Malfoy was still an unknown variable in her life at that point. He might make claims that he wasn't tracking her because he was ordered to do so, but that didn't mean he was telling the truth. They all had a thorough education in deception at the behest of their master. Telling a lie was second nature to his Death Eaters. There were times when they were forced to hide the truth from those too weak to understand that everything they did was for their best interest. Perhaps Malfoy was just utilizing his training.