They also don't only bottle 12 year whiskey.

The whiskey that they barrel this year, a portion of it will be bottled 12 years from now, a portion of it will be bottled 15 years from now, a portion of it 25 years from now, and so on.

They'll also barrel different portions in different types of barrels. So, of the 12 year whiskey, some will go in traditional oak barrels, but, if it's a larger production house, some in others types of cheaper barrels, and some will likely go in metal vats.

I assume that the ratio of those portions gets smaller and smaller as they get older. Like 50% of today's barrellage will get bottled in 12 years, another 30% will get bottled in 15, etc, to make whiskey more and more rare the longer it's aged. Plus, the longer you keep the whiskey in barrels, the more will evaporate.

All this plays into the price of the whiskey once it's on the shelves. With the metal-vatted whiskey being the cheapest because it won't have any taste, and because it will have evaporated the least.