You are assigned substantial amounts of reading and writing which may not be directly addressed in class. Testing is usually infrequent and may be cumulative, covering large amounts of material. You, not the professor, need to organize the material to prepare for the test. A particular course may have only 2 or 3 tests in a semester. Makeup tests are often available.
Teachers frequently rearrange test dates to avoid conflict with school events.
Professors in different courses usually schedule tests without regard to the demands of other courses or outside activities. Mastery is usually seen as the ability to reproduce what you were taught in the form in which it was presented to you, or to solve the kinds of problems you were shown how to solve. Mastery is often seen as the ability to apply what you've learned to new situations or to solve new kinds of problems. Consistently good homework grades may help raise your overall grade when test grades are low. Grades on tests and major papers usually provide most of the course grade.