Why did Minerva deduct only 20 house points for Draco’s nighttime wanderings but 50 house points each for the three Gryffindors’ nighttime wanderings? Maybe because Gryffindor is her house, and she is the kind of person to be harder on students of her own house than students of another house. Maybe she took off so many points because Harry is extra-valuable? Of all the kids in the whole school, let’s not lose Harry Potter, so let’s punish him extra bad to keep him safe!