Math can be cruel.
So, let's say you have a strong wind. Oklahoma-strong.
On your bicycle, you ride a tailwind and zip along at 20 mph. Unfortunately, you have to come back home fighting a headwind all the way. You manage to grind back in at 10 mph.
What was your average speed?
15 mph? You wish.
Answer: the harmonic mean of 10 and 20. This works out to 13.3 mph. Grrr.