Maybe BA is carrying more cargo as the policy is to maximise cargo in bellyholds as opposed to SIA's dedicated freighter fleet. Maybe we carry higher loads as I think we op fewer services to SIN. As for fuel reserves, doubt if there is a great deal of difference. We also always maximise thrust reduced take-offs. Other than that I can't comment on SIA's operating practices.
This thread shows there are many ways to skin a cat. Always assuming one wants to in the first place.