In light of Dave Reid's graphs I would like to modify my earlier car analogy.
Racing a car at the track up in the hills, I used to carry 40 litres of extra fuel in metal cans in the back, because the pump fuel prices at the circuit were exorbitant. The fuel I was carrying was cold and fresh, from a place where fuel was sold in large quantity and in presumably guaranteed quality. The fuel up at the track was expensive, thus not popular, thus not much used, thus of dubious quality, was the general feeling.
Reaches for hat and coat...