I got an e-mail telling me I was on a short waiting list as well yesterday, so I assume they sent out such a notifications to all shortlisted applicants though I obviously have no way of knowing. I have chosen to enroll elsewhere though, and if a few other people do as well then I guess they could expand their shortlist if it turns out to be too short.
The first paragraph of the e-mail reads:
"This email is to let you know that you are on a very short waiting list for admission to our graduate program. I apologize for the late notice. We have cut down our original wait list before sending out notifications, so it really is quite a short list."
in case you are curious. It is followed by some standard stuff (strong applicant pool, hope to admit you, please let us know if you are not interested).