Yes, it can be because of the stress of the exam. I think it's more that articles you read for fun are more interesting to you. Even if you're studying a subject you enjoy you're 'forced' to read about everything except what you really are wanting to. Which makes it hard to really dive into it and live in it. When you're reading for fun you're doing it by your own choice and you get the chance to soak in it. That makes it easier to remember it.