Most wines are a blend of some sort and thus consist of parts. The wine may be a blend of several varietals or it may be a blend of the same varietal from several different vineyards, different clones, or different lots. Those parts are often vinified separately. For instance, for a blend of Cabernet Sauvignon and Merlot, the Cabernet may be fermented and aged separately from the Merlot. The same may be true of grapes from different vineyards. The winemaker may keep them separated until just before bottling, because this gives her more control over what goes into the final blend and makes it easier to eliminate defects.
If, just before bottling, all the wine to be blended is brought together in a single tank and bottled quickly then each bottle will have the same wine in it, at least initially. But that is often not possible. Especially for a large cuvee a single barrel will not be large enough to hold all the wine and it will have to be divided up into several barrels, or the bottles will have to filled from multiple barrels in which the parts of the wine have been vinified separately. The problem is at that each barrel or tank is an individual that over time may develop its own characteristics. This is especially true if the winemaker is using native yeasts which can differ from barrel to barrel. Thus two barrels both containing wine made from Cabernet harvested from the same block in the same vineyard may have distinct flavor profiles by the time it is put in the bottle.
More importantly, wine also changes once it’s in the bottle. The characteristics of the cork, the bottle itself, and variations in how much oxygen is introduced into each bottle on the bottling line will influence the flavor of the wine as it sits in the bottle, not to mention differences in storage and transportation conditions.
So when you grab two bottles of your favorite wine off the shelf you may not in fact be buying exactly the same wine. Obviously, this is not a problem if the differences are small. The bottles are, after all, a product of the same grapes and winemaker and are thus part of the developmental potential of those factors that go into making the wine. In this case wine is a bit like a musical performances of a single work. Each performance is a variation of single work. But this variation does help to explain why bottles of the same wine may receive different scores from the same critic or why different bottles of the same wine give us different experiences.
For wineries and their customers who value consistency, minimizing bottle variation is a constant battle. But you know what they say about consistency and small minds. Perhaps we should accept the variability of wine as one of its most charming features.