"Waitlist" is a little bit of a misnomer.
At each Adcom meeting, there is some # of applicants that get accepted.
Following your interview, your file is added to the pool of applicants considered by the adcom. One of 3 things can happen. You can either be rejected (unfortunately, this does happen), you can be accepted, or you can be waitlisted.
Unlike most schools, "wailist" does not mean they are no longer looking at your file. At WMed, the adcom will review every file not rejected at every meeting. So let's say you interview in January. You get waitlisted because the adcom extended an offer of acceptance to these other 10 (or however many) people. In February your file is still in the mix. Let's say the adcom accepts 9 people that interviewed in February, and then they see your file sitting on the top of the list from people who have already interviewed and decide that you're the strongest candidate to accept this month. You'll be acceptance number 10. (arbitrary numbers here)
So all of that to say - the waitlist is constantly moving because it is not a "waitlist" in the sense that you're used to thinking of it. We're not writing you off until we run out of other interviewees. "Waitlist" at WMed means they want you, they reconsider you at every single adcom meeting and weigh you against every single applicant not rejected. At every meeting, they pick some number of people to accept. So really, in a way, everybody is on the "waitlist" (in that you're all equally considered by the adcom) until the adcom accepts you (hopefully!)
It might be better to think of it not as a "waitlist" but as a "qualified for acceptance" list. If you're not rejected, they want you and you're qualified for acceptance. It just may be that each month, someone is more qualified. That said, a number of people accepted for 2014-2015 spent some time "on the waitlist." SO if last year is any indication, being "on the waitlist" is generally a good thing.
Does that make sense?