To understand that, you first need to know where calories in wine come from. Basically, sugar and alcohol. Sugar or carbohydrates give your body 4 calories per gram while alcohol gives 7 calories for the same amount. This tells us that dry wines or even ‘brut nature’ are much better than those sweetened with ‘la dosage’. Before drawing out this conclusion, we must understand how the alcohol in wines is metabolized in our body.
Unlike other food items, alcohol is metabolized by our Liver, which converts it into acetate. This compound lowers blood sugar level and body starts deriving energy from it, rather than our fat deposits. As a result, alcohol adds empty calories to our body with no nutritional content. Moreover, the fat deposits remain untouched. And with wine, we do consume rich food.
Consequently, you end up amassing more calories with dry wine than with sweet ones.
So, when they say Low Calorie Wine, it is low on sugar and high on alcohol. Which implies that you end up earning more calories. In order to calculate an approximate measure of your caloric intake from a wine, multiply serving size and no. of servings 1.6 times.
For e.g. 5% alcohol wine, 6 oz serving, 5 x 6 x 1.6 = 48 calories
Add caloric intake from sugar to get the exact amount of calories from each serving of wine.
References  G A Colditz, E Giovannucci, E B Rimm, M J Stampfer, B Rosner, F E Speizer, E Gordis, and W C Willett. Alcohol intake in relation to diet and obesity in women and men. Am J Clin Nutr July 1991 vol. 54 no. 1 49-55