angel's share..

... or in this winemaker's case 'the cellar cut.'

The angel's share is the portion of wine (and whisky) that evaporates through porous oak barrels during maturation.
On this label it is referred to as the secret reserves of wine that 'mysteriously evaporates' from the best barrels in the winery to be enjoyed by the cellar hands.
While the former explanation is more scientific, the latter is much more poetic.