I have a different one, derived from teaching 1000s of students over 2 decades. the bimodal distributions comes from two sets of people: one set working hard and putting the hours in until the master the subject, the other set being lazy, cheating, and don't put the hours in, so they never achieve mastery. Appearance and substance.
Good test to tell them apart: if the lecturer mistakenly starts to talk about stuff that he already said in the last lecture (i.e. mistakenly doing the last one again), the first group will be happy and silent because there will be less stuff to learn for the test. The second group will tell the lecturer because they want to learn. This is then often mistaken by the first kind of people as trying to please the professor or "siding" with the prof instead of the fellow students. Because they think in a school-mindset.