How many interviews are people going on? What do you think is a safe number to match.
In general, the more interviews you go on, the better your chances of matching! This year, most people I talked to on the interview trail seemed to be going on 10-12 interviews. Most programs seem to interview about 10 applicants per fellowship slot.
The problem is that fellowship programs are much smaller than medicine residencies. The average # of fellows per year is probably ~5, with a range from 3-8 being filled in any particular year being the norm for most university programs. Given their size, most medicine residencies have a straightforward process of, "What are your clinical grades and evals like? How good are you letters? How much extra credit for research and intangibles? How were the interviews? -> Give score and then stratify applicants by score." The research and intangibles tend to be a relatively small chunk of the score. For cards, the research and intangibles are an awful lot of the score. Clinical competence is probably 25-50% of the score for cards depending on how academic the program, while it's probably 50-80% of the score for getting a medicine residency.
As such, every year there are more than a few applicants that go on ~10 interviews and don't match because their research and overall pitch didn't excite anyone enough to put them high enough on the rank list to match despite having good enough grades, letters, overall package to get 10 interviews. And, in the other direction, there are always more than a few people who only get a few interviews, but who go into them so hungry to match that they make an extra compelling pitch for how much they want to go to the program, how well they would align with it, write extra long love letters post interview, etc. You also have to remember to throw in a few less competitive interviews just because there does tend to be a crapshoot element at the high-powered academic programs which are generally all getting the same 100 very highly-qualified applicants and can end up narrowing their lists based on very small, subtle gradients of difference.
I do think the follow-up letters and overall level of interest demonstrated matters a lot more for fellowship than for residency. There are too many residency applicants for most programs to keep track of "interest" and use that to tweak candidate rankings, while there are a small enough # of fellows interviewing that most programs do make some adjustments for intense displays of interest. A lot of programs seem to also enjoy being able to say they didn't have to go very far down their rank list to fill.