The greater need for acid when using an SWG is from the increased aeration by the hydrogen gas bubbles and from some undissolved chlorine gas outgassing (especially if the pipe run from the SWG to the pool is short). You can reduce the effect of the aeration by lowering the TA level -- usually to 70 ppm if not lower -- and by targeting a somewhat higher pH range, usually 7.5 to 7.8.