As here (from Hereditary Genius, p. 21)
Every tutor knows how difficult it is to drive abstract conceptions, even of the simplest kind, into the brains of most people—how feeble and hesitating is their mental grasp—how easily their brains are mazed—how incapable they are of precision and soundness of knowledge. It often occurs to persons familiar with some scientific subject to hear men and women of mediocre gifts relate to one another what they have picked up about it from some lecture—say at the Royal Institution, where they have sat for an hour listening with delighted attention to an admirably lucid account, illustrated by experiments of the most perfect and beautiful character, in all of which they expressed themselves intensely gratified and highly instructed. It is positively painful to hear what they say. Their recollections seem to be a mere chaos of mist and misapprehension, to which some sort of shape and organization has been given by the action of their own pure fancy, altogether alien to what the lecturer intended to convey. The average mental grasp even of what is called a well-educated audience, will be found to be ludicrously small when rigorously tested.
Quomodocumque 
not very surprising for the man who coined the terms of eugenism and regression!
Just stumbled on this. His tone might be jerkish, but the substance isn’t. In college I discovered in myself exactly this phenomenon. I had a professor of physics who would give the most lucid and clear lectures on quantum mechanics, and I would leave feeling uplifted and energized by my new “insights” only to discover once I started on the homework that my understanding had huge gaps. Sometimes a bad lecturer can be a more effective teacher, because you don’t get the illusion of understanding, and you know you’re just going to have to struggle through the material yourself, though perhaps the teacher has still provided guidance as to what’s important.