So this was from the book "make it stick", recalling memory strengthens the network, which is why quizzing yourself or doing the simple sample problems at the end of the chapters are more effective then highlighting/rereading notes. The gap is mostly because you want what you've learned to settle into long-term memory where you can find it. Think of it as being a rat in a maze, you are on one side and the memory somewhere on the other, you want to work towards strengthening the path towards the memory, so it's easier to recall the next time around.