It also has a lot to do with the alcohol content too.
Water has a specific gravity (density) of 1 at standard conditions. Alcohol has a density that is less than water, so if the alcohol content gets sufficiently high (like it does in wine), then the density of the booze once all the sugars have been used up can drop below that of water.
I could probably explain this better, but I'm 1.5 sheets to the wind...
-Alex