The main reason is that although the 777-200LR has a longer range than the A340-500, because the 777 is wider it becomes less economical to operate an all business class service (which is always 6 abreast on SIA when it should be 7 abreast in Business on 777s). The flight is all business class which means that it is lighter, giving it a longer than specified range. Because the A340 has 6-abreast normally, more seats can be put in than an aircraft that is shorter which normally makes up for that in width.
Hope that makes sense although I'm not sure if I've explained that clearly enough.
BA 77