20He mastered Occlumency
Occlumency is not an easy skill for a young wizard to master. It involves closing your mind so that wizards with the ability to read it cannot do so. Although Professor Snape worked tirelessly with Harry Potter on Occlumency, it's something that Harry continued to struggle with.
However, Draco was adept at Occlumency
and picked it up fairly quickly. In Harry Potter and the Half-Blood Prince, Draco told Snape he learned to use it to keep the professor from learning about his plot to take out Professor Dumbledore. Snape later surmised that Draco learned the skill from his aunt Bellatrix Lestrange. It makes sense that this was one skill that Draco excelled at, where Harry failed.