Once you hit the passing grade, you're done with the test. It's over. You passed. Those 9 questions left don't matter anymore. So if you don't know the answer, skipping it lets you potentially get a different question in its place, letting you pass without ever having to answer it. Unless you're getting that many wrong that it comes down to that question at the end. Only answer what you KNOW is the right answer, don't take any guesses until it's repeating the skipped questions.