Wind resistance is a non-linear function of the windspeed relative to the rider (if the wind speed is doubled, the resistance increases by a factor of eight, or something like that).
In addition, the time taken for a given distance is a non-linear function of the average speed. If a journey at a speed S takes 30 minutes, going twice as fast reduces the journey time by 15 minutes, but going twice as fast again reduces the journey time by only 7.5 minutes.
These two effects mean the gain from the tailwind is less than the loss from the headwind.
