It doesn't say cubed, it says cubic, not the same thing IIRC my Maths class. One is a power, the other is a geometric description.
We refer to a mile square (A two dimensional shape the lengths of which are defined by a one dimensional unit), I have no problem referring to a cubic acre (A three dimensional shape defined by a two dimensional shape)! A cube where the surface area of each face is an acre is a cubic acre. Three cubic acres is three times that volume.