Basically (from what I understand) is that the school has technically already sent out 70 acceptance offers to people, so interviewers are interviewing for a pending open seat spot. This means that the faculty and stuff believe you are a good fit for the school and, given that someone who has already been offered a spot declines their offer, you will get an acceptance offer. The waitlist is a completely separate list from the pending open seat list. Offers will be given to pending open seat and once everyone on that list has been given offers, only then will offers be made to waitlist individuals.
Again, I could be wrong, but that's how I interpreted it when I interviewed. Hope this helps!