There's only one year in between a medicine intern and the medicine senior, and an "upper level" medicine resident might only be a PGY-2. That's pretty close. If you consider the surgery programs where someone can do research years as a resident, they might be a PGY-1 and a PGY-7, which is a much bigger difference.
Plus, the roles in surgery do actually change quite a bit. Now of course, the interns can't take overnight call at all, so that's one thing. The other big thing is that you don't operate much (or at least not on big cases - if you do a lot of cases, it will be hernias, anorectal cases, etc) as an intern, and it's a gradual ramping up.