Well, why not? Starships have really funny shapes sometimes - why the curved pylons on the E-D when there could be simple straight slabs instead, say? Possibly exterior shape does matter (warp dynamics or some such nonsense), and one just must
have that beautiful undercut in the secondary hull. So what's one to do when there's not enough stuff to fill the hull with? Create the undercut shape with hollow, walled space, I guess.