This phenomenon happens regularly. First some background information. In this department we give first-year students a placement test and divide the group into three smaller classes based on the test results. Then each of those groups is split in two to make "Conversation" classes based on their student number.
Today I taught Conversation class, and as always, one of the classes is far superior in attitude and ability to the other class. I will not say which is which for privacy purposes, but I am amazed by the difference. I can think of several possible reasons; time of the class, group dynamics, people with higher student numbers behave differently from students with lower numbers. I start to cover the same content in both classes, but end up doing much more in one than in the other. What is it?