An aft CG reduces the download on the tailplane, or possibly even creates a small lift on it. Any download on the tailplane has to be countered by increased lift on the wing, which increases drag, so aft CG saves fuel.
Of course they could have designed this out altogether, but aeroplanes are left like this just to create another conundrum for generations of aerodynamicists and pilots on interviews to have to answer. The 'A' Level question then becomes: 'what are the problems created by having an aft CG?'