M2 here.
So, Holmwood, I love ya but you've jumped a little beyond your knowledge (even though you sound very confident)
There are a good number of acceptances post interview. SDN does not represent all applicants, may not even represent most applicants. So remember that you don't hear the entire story.
Also, some interview days have more acceptances than most. This is because technically, post interview, EVERYBODY is evaluated by the adcom. The people who interviewed at the most recent day, and all the ones who interviewed before that. So everyone is evaluated, and then some acceptances are offered. Sometimes, most of the acceptances come from the most recent interview day. Sometimes, people from previous interview days are stronger candidates. The entire interview pool is evaluated at every adcom, and the most competitive applicants from that entire interview pool is accepted.
Doing this has nothing to do with whether some applicants see us as a backup school (and I think that most accepted people don't hold that view, although I could only offer anecdotal evidence). This approach was decided before there were ever applicants. It's a slow and careful building of the incoming class in order to make sure that early interviews aren't disproportionately accepted, and to make sure we don't miss exceptional applicants in later interview days.
Post-interview rejections definitely happen but I don't know if I would call them "common." If you got to the interview, chances are we want you - that's typically not the question. The question is, of those candidates we want - are you the best candidate for the seat available?
Consistently, current students comment on how cohesive the class is, how everyone is on a similar level, and how well we all work together. This is the result of incredibly hard work by our admissions office to make sure that the absolutely best candidates make it through our doors. I realize that can make it feel like we leave you waiting - but I promise you, you are being considered at every adcom meeting.
And finally - responding to another previous comment - year 2 and 3 are not flipped. You can see a curriculum map in the student handbook (available online to anybody), it's a standard progression from basic sciences to organ systems in year 1 and 2, to clinical rotations in years 3 and 4. We take step1 at the end of year 3, like students at a number of other schools (including Penn, FIU, Columbia, NYU, Baylor, I've heard Hopkins is switching to end of 3rd year soon). The average step1 score goes up when schools switch to end of 3rd year, so we're following what appears to be a best practice on that.