I was told by one of the Western program coordinators this morning after being notified of being on the waitlist that offers off the waitlist are in part based on subgroups that they place each waitlisted candidate in. Some (perhaps most) schools follow a numbered waitlist, while Western MPH's program does not.
She explained that I am placed in the "undergrad science" waitlist, and similarly, others are placed in "undergrad arts" and other similar subgroups. As people either accept or decline their offers over April and May, they will be making sure that their incoming class is diverse enough by pulling from the waitlist groupings they need. I'm assuming that, for instance, if they have too many "undergrad science" students accepting offers and making up their incoming class, then they'll pull from other waitlist subgroups.
This being said, I think by rolling admissions they mean that they are evaluating their waitlist individuals on an ongoing basis depending on how their class demographic is looking. I also want to note diversity may only be one instance where they pull from waitlist groups - there could be many other reasons as well!