Water content of salted butter
The FDA definition for butter specifies only that it be made exclusively from cream or milk, contain a minimum of 80% butter fat by weight, and may include salt and coloring.  

FDA Butter Definition      

USDA Butter Standards

If you assume the butter in question is at the low end of the spectrum with exactly 80% fat and equal amounts of milk solids, then unsalted would have less water than salted.  Since there is no standard that says milk solids must also be a specific percentage, or that fat content can only be 80% and no more, there is really no way to accurately make the claim that salted butter always contains more water than unsalted.