We have been marking exams. We have noticed that students, when learning a subject and making notes, take a large quantity of input from the Lecturer and from the text books and the web and from other sources, and then precis it so that they can more easily memorise the salient points. This process we call "Compression".
Now, when it comes to an exam, the typical question consists of 3 or 4 paragraphs, or sentences even, and the student is expected to produce several pages of expanded and carefully worded answer chosen judiciously from what (s)he has learned previously. This is the converse process, from a small amount of information in the question to a substantial answer. We call it "Expansion".
Very often this procedure results in acceptable and satisfactory answers, to the examiners. Even if the answer does not agree exactly with the pro-forma "answering scheme", the examiner may recognise the truth and relevance of what has been written, and may even learn from the extra insights offered. However, there are lots of ways this can and does go wrong, particularly with people who are not Native English speakers (and I include most if not all US people in this). What may happen is that the student doesn't realise the degree to which expansion is needed (over-long answers are commonplace), or that (s)he produces a body of knowledge which the examiner does not accept as, or recognise as, an "answer to the question".
The result is bi-modal mark distributions, in which, from the examiner's viewpoint, the student is either "mostly right" or else "mostly wrong". Now, the democratic process tends to lend more weight to majority opinion or views. Thus "mostly wrong" may become promulgated and then redefined to mean "mostly right". In this way, serious error (from the examiners' or originator's point of view) creeps in to efforts to create an acceptable (to the originator) body of knowledge. (It is also noticeable in the formulation of democracies' foreign policy. The result is that the "democratic tail wags the dog" and the policy formulators find that they have unleashed forces which they had not intended.)