That is because you fail the test questions. In a task, there are questions that are set (test questions) to check your accuracy and ensure that you are not just clicking through. You need to follow through the instructions to ensure that you do not make a mistake in any of the questions asked. Sometimes they put only 3 test questions in a task. If you fail one, that is 1/3, which is 33%, and since most tasks require about 80%, you are expelled. If you fail two, that is 2/3, which is 66%. You are still expelled because you fell below 80%. So for such a task, the only option is to pass all three test questions. This is just one scenario. Sometimes the test questions are more. The important thing is put in your effort to pass EVERYTHING.