Not all wineries store their wines in perfect cellar conditions. More wine gets moved around than you might imagine, and that can greatly impact the quality of the wine, especially as it ages.
When several bottles of 1996 Cabernet from two prominent Napa Valley wineries tasted oxidized while I was working on my '96 retrospective report, I asked the owners-winemakers how the wines had been stored. I’d had their wines on numerous occasions and wondered why the wines they had submitted were expired.
They both came back with the same explanation. Upon reviewing their inventory records, they found that the wines they sent to Wine Spectator's Napa office for the tasting had come from suspect cases. In both instances, the cases had been returned to the winery from retailers in exchange for a different vintage. (If retailers order too much of one vintage, they are allowed to exchange that wine for another year).
One winemaker, who logs which stores his bottles go to on a computer, was actually able to go back and try various bottles from the returned case. He discovered that the wines were tired, which probably means that they were improperly stored at the shop or during shipping.
The problem was more about the way the wine was stored than the actual wine. When the winemakers submitted new wine for me to taste, they were relieved that it showed considerably better than the wine they originally sent.