Typically concentrations are given in w/w (weight/weight). When it comes to weight/volume things get tricky, especially when you use non-metric units. In SI w/v is reasonably simple and for most cases where approximate concentration is enough grams per 100 mL of water solution (which is how w/v is typically understood) are equivalent to w/w. When it comes to tbsp/gallon things get so convoluted it doesn't make sense to use w/v, as you need some additional conversion factors.
Sorry, I never thought about tbsp as a unit of volume, more like "1 tbsp is about 17g of salt" (actually that's for a kitchen salt, not for calcium hypochlorite, I believe it should still keep you in a right ballpark, although just weighing the hypochlorite would be much better). My bad for no stating that clearly.