I already outlined an example above.Is there actually evidence that MGH residents have better outcomes than crap-tier residents outcomes?
Having more extensive prior experience in complex surgeries leads to better outcomes with those procedures. MGH does way more complex procedures than a rural community hospital would, so residents there would have more exposure. This concept extends to other specialties. Community hospitals (not all, but many), when they don't know what's going on with a patient, send them to places like MGH. Therefore a resident from MGH will have more knowledge of unique pathologies and clinical presentations. The more you know, the more you can manage as an attending.
Experience is important