Because a lot of people are still holding on to multiple acceptances right now. So if they have 93 people that said they will come for NOW, then they will not overaccept like some schools do. Instead, as people start withdrawing, more will be accepted, presumably from the new interviewees or the holds (someone got off of hold already, so they are taking people from that list).
And in response to Bruin, the waitlist is generated after May 15th. The newly vacated spots from those who withdrew after May 15th will be filled first from the HOLD LIST. Then those left over from the hold list will be either waitlisted or rejected. They will continue to draw from the ranked waitlist as people withdraw from the class up until the day of class in early August. I guess starting early August is one drawback for those waiting for a spot since once school starts, no one would be taken off anymore. We started in late September last year and some of my classmates got in during September.
Come May 15th, quite a few more spots will open up. A few people from SDN alone got in during that time period last year. I had to wait a little longer than that. Good luck folks!