Reversed angel´s share?

[Opi]

I wonder if anyone knows the answer:

In some bourbon and scotch (e.g. Springbank I heard)distillery warehouses the relative content of alcohol in the casks rises during maturing (when its hot inside the warehouse during summer). How does it works?

Alcohol definitely evaporates better than water and the oils, so it must have been the wood of the cask.

[Ize]

IMO, it sounds very peculiar that wood would make the difference. I would assume it (if this reversed angel's share really exists) to come just from ambient temperature and humidity.