Intruder is correct ONLY if the same speed is used to calculate onward and divertion fuels. As BIK pointed out, if the aircraft is to be flown at best range or best economy in either case, speeds and therefore fuel burn will be different depending on wind compoment.
If you were manually working out an ETP, you would likely use the same IAS and TAS for both sectors, but I would imagine a sophisticated flight planning program would use LRC or BEC and therefore come up with different speeds.