Most whisky is reduced with water before bottling. That is not an adulteration but a consistency measure: casks vary, and reducing every batch to a fixed figure is how a bottling tastes the same from year to year.
Why the number is never round
Strength changes in the cask over years, and which way it moves depends on the climate. In Scotland's cool damp air, proportionally more alcohol than water evaporates, so strength falls. In hot dry conditions more water leaves, and strength can rise. A cask-strength bottling reports wherever that particular parcel ended up, which is why the figure is oddly specific.
Adding water is the point
A cask-strength bottle hands the dilution decision to the drinker. Water lowers the strength and also changes what can be smelled: reducing alcohol releases aroma compounds that were bound up at higher strength, which is why a few drops can visibly change a whisky.
Haze is not a fault
Cask-strength whisky is often not chill-filtered, so adding water may turn it cloudy. That is fatty acid esters coming out of solution — cosmetic, and the reason chill-filtration exists at all.