From what I've noticed, the more you understand the subject, the better you learn from reading on your own and the less you learn from exercices, listening or conversing. I think the cause is that there is a difference in focus: With books, you can identify the things you already understand ( as-is or as an analogy to something you understand ) and focus on the new stuff, so the more you understand, the better you focus on the new stuff. The problem is that when you don't understand lots of things well enough, your concentration is not focused, so you end up with a superficial understanding and forgetting a lot as you go. The solution comes in the form of exercices, which provide an artifial focus, having your full attention until you understand them very well, which is good if you're a beginner, but redundant if you are experienced.