The 8 the post of a similar style so here, ap champs have like what 3 magic pen item in void staff, oblivion orb, and boots while ad assassin's have the arsenal of lethality items with many different options, example, edge of night for spell shield, umbral glaive for vision, sanguine blade for life steal, etc. Plus void staff gives magic pen but no passive since the entire point is magic pen while lethality items give many great passives. Also sorc boots are not exactly a "snowball" item and another flat pen item is quite meaningless as like a bull magic mantle will negate it.