Arguably, cask strength Whisky gives license to the drinker to dilute their own drams, but in-house distillation which will be sold at around 40% ABV, should also capture the Whisky’s optimum flavor profile.
For aspiring mixologists, cask strength Whiskies include Aberlour’s 69.1%, Bruichladdich’s undiluted 70-72% and Port Charlotte’s bottling around the same ABV mark.
When diluting a Whisky, a distillery has to take into consideration the angel’s share of alcohol that has been lost to evaporation during the ageing process. This is usually between 0.5 and 1% every year, so with every year of aging, a Whisky will lose a little of its alcohol.
Dull but true; measuring the ABV of a Whisky is also a practical issue to help the producers. Back in 1970 Scottish malt Whisky would still tend towards 63%, whilst Irish Whiskey was around the 71% mark. By making a uniform standard, casks are easier to pass around between different blenders.
Not to mention that if you dilute a Whisky, you get more from one cask. During WWI the maximum bottling strength was temporarily brought down to 37.2%.
Single Malt Whisky bottling can have various ABVs, as they will often include various Whiskies from the same distillery. Each cask in each distillery has its own microclimate, so getting an across the board ABV into one bottle is easier said than done.