Many teachers have certain expectations and make a lot of assumptions in the way they believe their students will do a particular assignment. Sometimes, however, students have their own methods and procedures that may cut out entire portions of the expected trajectory. Students may lose valuable information, but there is also the chance they will not, after all, most teachers tend to do reviews and discussions of the information students were supposed to read or research.
Speaking as a student, myself, I very rarely follow the path my instructors would like me to. It is not because I think the information is useless, but rather because I know that the information will be gone over at some point and it would just be redundant to bother with it (plus, my time could be used on other responsibilities).
Here is the typically expected trajectory of reading and writing versus what I actually do: