The aim of the CAPM test is to give you an introduction about project management, so you can understand the basic concepts. That's why the CAPM stands for the Certified Assistant in Project Management.
The aim of the PMP, on the other hand, is to ensure that you know how to manage projects. It doesn't focus on how well you have memorized the project management terms, or how good you're familiar with project management. The PMP tests your abilities of managing projects by focusing on situational questions (An example question would "One of your key team members resigned in the middle of the project, what would you do?"). That's why the PMP stands for Project Management Professional.