Because the classroom as it is to learn that according to subject or as a lesson, but the actual work is included in many subject lessons gathered together, which are even more complex.
Due to classroom learning, it is isolated by topic or lesson. But the real work is to gather together many lessons, which are more complex. And some operations may be different from what I have learned.
Since the classes it as learning separate topics or according to lessons. But the real work is to collect several lessons together, which have become more complicated.