Because your situation is:

1. Wild yeast in grape skins converts sugar into alcohol. When you add sugar, yeast will convert more sugar into alcohol. Thereby increasing the alcohol content.

2. When the alcohol concentration reaches a certain level, the yeast will die, and the sugar will not be converted into alcohol by anaerobic respiration, so the sugar you add improves the sweetness of the wine.

Since you think your red wine is sweet, it is because of "the sugar in the red grapes you use" and "the sugar in the white sugar you put in". Compared with the number of yeast, the sum of these two amounts is too high! The relative here can also be understood as the relative sugar content of yeast is too small!

Conclusion: Your explanation makes sense! The grapes you bought are so good and sweet!