Here’s an odd truth for you: you never really know how good a wine tastes, even after you’ve tasted it. The only way to truly understand whether a given wine is any good, and to evaluate just how good it is, is to try it many times. The reason? Something called bottle variation. It’s not some insidious thing like cork taint, but subtle and even large differences can exist between two seemingly identical bottles of wine, as any perceptive person who regularly purchases wines in larger quantities can tell you.
Most pronounced the older a wine gets (age tends to magnify bottle variation as well as simply provide more time for things to happen to a bottle), differences in bottles can be due to factors in how the wine was made and bottled, how it was shipped, or how it was stored.
The more wine I drink, and especially the more I sample older vintages, the more I realize how often there are differences by bottle.
In any case, Dan Berger has written a simple overview of the subject today in the Napa Valley Register that is worth checking out.