So as far as I can tell they do go back previous months. The way they do it is they rank you the day of your interview and you go on a list, and the top of that list gets accepted each week. Usually its 2-3 get accepted each week (2 in state 1 out of state) from that interview week, and by the end of it all they are able to fill the class that way. You after your interview will get a letter that says something along the lines of accepted, denied, or "we will consider your application at a further date". That hold letter is not the waitlist, which is kinda confusing. From my experience most people that get the hold letter wont actually hear until the waitlist goes out, but some can depending how good their interview/application was. And the "good" is generally people that rank higher than the previous years rank for the class. so like if the average is 7/10 they will accept you if the board scores you at 8/10 or something (i'm not 100 percent sure that's the actual ranking, but my mentor who was previously on the board explained it to me that way).
For eagles doc, in March/April time frame Shelley will send an email notifying you of your placement on the waitlist. This is usually after a large round of acceptances. Then, once again from my experience, there is a LARGE acceptance about a week after the waitlist goes up because people get acceptances to other schools that they would prefer.