It's not uncommon for Moodle courses with first-year or transfer students on the roster to show a discrepancy in enrollments relative to the official Banner roster. This is because students do not have Moodle accounts until after their first login, and those two groups of students typically don't log in until they arrive on campus.
There are two possible solutions. The first is to wait until your first class meeting, and remind your students at that time to log into Moodle. The second is to email the students directly and ask them to log in immediately. In either case, the Participants roster will be updated at the next daily data sync.