Surprised: Not my experience, Paul.

Am very fond of the taller balloon flowers and have used them for 15/20 years.

Especially like the Hakone series (the only doubles I'm familiar with).

Have never seen any (reversions?) to single from double flowers, even though I find balloon flowers are frequently very long lived in perennial beds.

Our front garden (July 27, 2013):

I'm working off of memory here, but on another gardening forum some time ago, a gardener told of how her balloonflowers would first bloom double in the spring, then the reboom would be singles. The following year they would repeat--double, then single. Maybe it is a response to an environmental factor, such as higher heat or some other stress. Or maybe the plants were just too tired to fancy up their flowers with extra petals the second round. ;)

Even more factors can come into play from one year to the next. And reverting has varying causes among different plants; it is actually quite complex, having the plant determine what those cells are going to produce. Maybe the particular strain you are growing is less stable with respect to doubling.

Funny this should come up, I noticed it on my 'Astra Semi- Double Lavender' balloon flower the other day.

Perhaps certain cultivars more prone to this? I never noticed it on my Hakone Double White when I had it.

Here is the interesting thing the ASDL does- little curly-q's at the base ;-)
CMK