My suspicion is that such results reflect a failure of teaching.

Imagine that you are teaching people mathematics, and you skip some beginner lessons, and start with the more advanced ones. Some people will have the necessary knowledge (from home, books, internet, etc.), so they can follow you and improve their knowledge. Most people simply don't understand what you are talking about. At the end of the year the test will show that there are two separate groups -- those who know a lot, and those who have no clue.

Please note that the failure of teaching is not necessarily at the level where the problem was discovered. It may be a failure from previous levels. For example a university teacher may expect some really simple knowledge, but many high schools fail to teach it.