The difference you're seeing is all about density, which cooks call "bulk density." The salt itself is the same, but the crystal shape changes how much you can fit in a spoon. It's like the difference between fine sand and coarse gravel.
Fine table salt crystals are small and cubic, so they pack together tightly with very little air. One level tablespoon is about 18 grams.
Coarse kosher salt has larger, irregular flakes that don't pack well, leaving a lot of air. For a brand like Diamond Crystal, a tablespoon is only about 9-10 grams. Morton Kosher salt is denser, about 14 grams per tablespoon.
For bread, the salt percentage is critcal for controlling yeast fermentation and gluten structure. If a recipe gives grams, it's because that precision matters. Your best bet is to get a cheap digital scale. It will make all your baking more consistent and true to type.