There are many classes at the university that have multiple components (lecture, lab, discussion, recitation, etc). These components each have their own section number. If you are assigned as the instructor to the lecture component only, you will only see the class roster for the lecture. If you are assigned to one or two labs, then you will see the rosters for those sections and not the overall roster.
Changes to this setup can be performed by your Department Scheduler.
This allows you to download class rosters for either the entire lecture or for each individual's labs, discussion sections, etc. You can also send an email to the entire class or to one section of the class.