Alternative explanation: They're shutting up and multiplying.

Most people have gone through the education system. Most people know how to guess the teacher's password. Most people have learned better than to assume their answers will be counted correct just because they have (in their opinions) good reasons for holding those answers.

Does putting an incentive on getting the answers "right" lead to "right" answers, or does it lead to people answering the way they expect you to treat as being right? My own educational history suggests the latter.