Watkins urges dealers to superchlorinate with at least two ounces when filling a new spa. They wet test 100% of their tubs, and even though they take steps to vac out as much water as possible, they can't get it all. During shipping, sometimes the remaining water gets a bit funky. Sorry, but your spa is only new once, the 2-3 ounces refers to that one-time first fill.
Water changes are very different, and a small amount of chlorine is usually fine: The tub most likely has good water left in it, and most tap water brings in it's own load of chlorine anyway.
HTH