At first I wondered if it was cask strength... apparently it is.Seems odd the 52 year is only 82 proof
As tif pointed out the storage conditions are what drive the alcohol content over time. Every cask of aged spirits is constantly losing ethanol (alcohol) and water, but water and alcohol molecules are different sizes. Water is relatively small compared to ethanol. Due to it being so small, the water can migrate through the cask at a much faster rate than alcohol can. Not only that, trees are sort of designed so that water can migrate through them. Ethanol? not so much...
So with both water and ethanol leaving the cask, what causes scotch to either stay steady/lose strength while american whiskeys get stronger? Climate. In order for water to leave the barrel, it needs to evaporate. In a cold/moist environment like the coasts of Scotland, this is a relatively slow process so the loss of ethanol and water are relatively equal, or ethanol leaves at a slightly faster rate causing the alcohol content to go down. In drier hotter conditions like Kentucky or Tennessee, the water leaves relatively quickly compared to the ethanol causing a relative increase in alcohol content.
Is this why something like bourbon is bottled as 'white dog', which is like isopropyl abv, cut with water to dilute, knowing it will proof back up over time?