Fox Moth I will try to explain why an increase in headwind is worse than a decrease in tailwind.
Take a 4000nm sector at TAS of 500kts would take 8 hrs still air
Take a 4000nm sector TAS 500kts wind ahead at 100kts takes 10hrs a difference of 2 hrs
Take a 4000nm sector TAS 500kts tail wind 100kts takes 6 hrs 40 min at difference of 1hr 20 mins
Thus a greater headwind has a proportionally greater cost than a lesser tail wind of the order 3:2.
I hope that is clear
HF