It is identical to the system used to grade school children's homework all over the United States. With the notes, came scores that were reproduced and quoted in place of the lengthy descriptions. The product evolving in the bottle may turn out better or worse than when the wine had been assessed. Take the case of wines in the barrel. A well-known critic had assigned low scores, panning the 1994 Bordeaux before it was bottled to find that after bottling, almost all wines of that vintage were evolving into wines of distinction.
Tasters are human.
This involves statistical analysis to ascertain if a taster, tasting the same wine at a different time and in a different order to other wines, would return the same scores which are also consistent with everyone taking part in the taste tests. It is also vital to consider the description of the wine's style, personality and potential. However, there can never be any substitute for your own palate nor any better education than tasting the wine yourself. The most glaring danger to wine is that of supremacy - the overall reliance on one critic's scores when one palette and one stylistic criteria of a critic turns into the industry benchmark. In such a case, the most popular critic's criteria become the major obsession of the winemaker who wishes to please in order to assure success of his wine.