There is a pool of questions for every topic. When you take a quiz, a question is selected from the pool. There is some programmed logic deciding which question to choose, which also depends on your score. For example, you get the questions where you choose the answer from a drop down list early on while the free form text questions come later in the game.
To raise your score you need to retake the quiz for the specific question. Every correct answer will increase your score and every wrong one will lower it.
