Why can't Mephisto drop super rare unique items?
Hell Mephisto's monster level and treasure are completely different. His monster level is 87, but he doesn't drop anything from TC 81, 84, and 87.
Hell Meph drops up to TC78, while NM Meph drops up to TC54.
Atma's drop calc is obsolete. Use this one: http://mfb.bplaced.net/dropcalc/dropcalc.php?lang=en&patch=113&mode=lod&interface=default&window=true It is mostly correct, just not for duriel, countess and some super unique monsters.
You can see when you select Special Monster -> Mephisto -> Hell that his monster level and TC are different.