@konsole: You are comparing Dusk Sword which has SA: Acumen with a weapon that has SA: Empower. If you really want to compare things, compare Elemental Sword: Empower with Imperial Staff: Empower, or Sword of Miracles: Acumen with Dusk Sword/Staff.
Exactly what Verena said. Dusk items are meant to be a bit more powerful than S Grade items, but not insanely powerful.