<p>Perhaps there are seats reserved for students of a specific major, class standing, or class level, which the waitlisted students do not fulfill?</p>
<p>Could be just a delay in enrollment. Sometimes people drop a course, but the waitlist doesn’t update the class enrollment. You just need to wait it out and eventually the waitlist will work making the class 30/30 with 3 waitlistees.</p>
<p>It could also depend on the class. Some of them are ones where the Professor approves the student entering so they’re waitlisted until they’re approved.</p>