Even among different brands of Kosher salt there are differences. For example, the Morton's Coarse Kosher salt, the brand I use, weighs about 7.7 ounces per cup. From what I have read, the Diamond Crystal brand of Kosher salt, which is the major competing brand to the Morton's Kosher salt, weighs about 5 ounces per cup. By contrast, ordinary table salt weighs around 10 ounces per cup. Even with those differences, the instructions on the package of Morton's Coarse Kosher salt say to use it measure for measure for table salt. It's far easier to say that than to tell users to use about 25% more of the Kosher salt. It's unlikely in any event that one will measure it out that precisely using ordinary measuring spoons.
You will often read instructions that say to use 1 1/2 to 2 times Kosher salt for table salt (on a volume basis). That would work for the Diamond Crystal brand of Kosher salt but not for the Morton's brand of Kosher salt or other like brands.