I don't operate the 340 but would assume it's the basic aerodynamic principle of elevator/stabiliser downforce. The THS would be have less downward deflection with an aft CofG. It is the same reason that you will get better fuel economy with an aft CofG.
Check out 'Getting to grips with fuel economy' --- think you can download it from the Airbus website - else your company should be able to provide you with a copy.
