Last year the ACGME started a semi-crack down on this practice and is strongly encouraging/demanding that programs stop sending these sorts of letters, because sometimes they were being interpreted as false offers of positions,etc, or else they were swaying people's rank lists in odd ways. I think there is variation in how much programs are respecting this shift, but in general there is supposed to be less of that coming from programs than there was in the 2011-2012 and earlier. So you may be a superb fit but just happen to be applying to programs that really are going along with that change.