According to
http://www.aopa.org/whatsnew/newsite...0317avgas.html

"Avgas often is stored for long periods of time..." and
"avgas is not as affected by market fluctuations in oil prices, unlike
automotive gasoline and jet fuel...."
read more.....

The implied info is that due to its small production avgas is not made
so often as other fuels and the stored quantities last for a long time.