Here's why:
* Starting Points: All houses begin with 0 points at the start of the school year.
* Points Earned: Throughout the year, Slytherin earned points for various achievements, including:
* Draco Malfoy's cunning tactic to get Harry expelled, earning them 50 points.
* Snape's biased judgement favoring Slytherin, awarding them points for questionable actions.
* Points Deducted: Slytherin also lost points:
* For Snape's reckless action of allowing a troll into the castle, which resulted in a significant point deduction.
While the exact number of points awarded and deducted isn't specified in the book, the final score of 170 points is confirmed in the movie.