Look at it in two parts if you like.
The actual ATPL material and the progress tests: these test your understanding of the subject matter. You have to really grasp the subject to pass the tests. All prog. tests are submitted to the CAA to show you meet the required standards.
The CAA exams: many of the exams do not require total knowledge of each subject area, quite simply because there is so much material! Go to the residential ground school, absorb the intensive two week brush up and then hit the OQB and the feedback papers. Keep hitting them. Refer back to your notes if you absolutely need to, but just go to town on the OQB and the feedback. By the time you come to the exam, the questions and content should be second nature to you.