I think your understanding works here Keith Bradford.When I studied education, we were often told: "If they haven't learnt it, you haven't taught it."
I guess this quotetion means the same: "What you have taught is what the students have actually learned; this may not be what you think you have taught."
Possibly, it means that we are not truly masters on a subject unless we are able to teach it. I've heard around that when you teach something to someone, you retain the information better, and also witness the different ways in which different people go about learning.What does it mean: "What is taught is what is learned."?