As explained, it's the concentration. If you don't have a dishwasher with a water softener: You don't sprinkle a little salt in some compartment, it's large and filled with 1-2 pounds of salt to make a saturated brine. The salt sold for dishwashers is purified NaCl (table salt has some other stuff like anti-caking agent). That's what the dishwasher uses to drive the Ca2 and Mg2 off the resin.