Winemakers may add sugar to wine to “improve” the taste, and sugar content can vary widely.
Source — nytimes
Q. Some wineries add sugar to dry red wines after fermentation so that they taste “smoother” to the American palate. How can I find out how much sugar is in what I am drinking?
A. To find out how much sugar might have been added to a given wine, your best bet may be to contact the producer directly.