It's a little more complex than most really care for, but a lot of it comes down to how the wine is made..
Every batch is different, because of the different levels of flavor, alcohol, acidity, sweetness, tannins.. We strive to balance these different facets, but some batches, while 'balanced', are 'more' in every aspect than other wines (the ones with intense flavor make the best long-lived wines, imo).
Even though we may find a balance, and bottle the wine.. It's not truly done aging until you've opened a few bottles (spread usually month after month) and the just-opened bottle tastes like the last bottle did - and it's what you'd expect from that bottle. When you repeat that process a few times in a row, its safe to assume the wine is done aging.
From that point, however old it is, it's 'safe to assume' that the wine will keep for that-much longer.. So I opened my Peach at 18 months, and it was done aging, so I assumed that it would be alright until it was 3 years... It's now over 3 years old, and still decent
And then there's also corks & wax. Some corks - basic agglomerated corks, only hold up for so-long (18-24 months if i remember) because they allow O2 through faster than higher grade corks (1+1, or grade 3, 2 or 1).. Over longer periods, the lower-grade corks can also fall apart when uncorking, even if they've been waxed to save the wine. Wax will seal off the atmosphere but shouldnt be done until the wine has actually finished aging; prematurely waxing will stop the aging process