Low number of butterflies "a shock and a mystery"

The Guardian reports:

If you think you saw fewer butterflies than ever this British summer, you are probably correct: the Big Butterfly Count has recorded its lowest number of common species since records began. 

Normally ubiquitous butterflies such as the gatekeeper, comma and small copper experienced their worst summers in the history of the count, which is run by Butterfly Conservation and began in 2010. 

Scientists said the low number of butterflies is “a shock and a mystery” because this summer was warmer than average and much drier in England than the previous worst year for butterflies, 2012, which was unusually cold and wet.

Because I approve of butterflies, I have a buddleia in my front garden, but I hardly saw any this summer.