It's possible that the program really wants the applicant they fedexed the letter to. However it's also possible that applicant is ranked anywhere on their list and they just sent it out fedex to ensure timely delivery prior to rank list deadline because it was in the last group of letters they sent out. Or the secretary could have sent it out wrong. Who knows, the point is we won't ever know with certainty.
Writing those letters takes time. I received a very personal one from Zapol when I interviewed last year. I interviewed there fairly early in the season and my letter came shortly afterwards. Based on the content it was definitely written by Zapol. I find it extremely unlikely that he would waste time writing all these letters for applicants that they weren't going to rank. And if they're ranking you, there's a chance you could go there if you rank them. Remember that programs want to get the best residents they possibly can, and the match favors the applicants preference so programs do things to lure you and tip the balance however they can. MGH is a great place and program, but many people are intimidated by the name. A presonal letter from the chair can go a long way to sway someone about the program and they know this!!
It's really difficult, yet important to rank programs in the order that you want and is best for you overall. When you are looking at top programs really ask yourself if you are getting sucked in by the name of a place and if it is truely a good fit for you. Certainly top programs will give you great training, and there's all different styles in programs, but programs aren't one size fits all. I almost got sucked in by the name of a place and it would have been a big mistake for me. Luckily I made the right decision in the end, but it could have very easily gone the other way.