I agree, the Admissions Office is a little brief on the phone. When I had called they told me that 'several hundred' people were on the waitlist but that the number was decreasing as time went on, and that they would take 'a couple' from the PWL. I don't know how many that means but it would make sense that they might have been slightly under-enrolled.
FYI, sent my app in December got PWL about two weeks later, so I doubt they even read my application at that point. They probably wanted to be conservative with their admits so they could increase their yield pulling from the waitlist, I think.