Is it possible that the sodium released by the salt cell is being bound by boric acid as it leaves the cell? It seems like that would increase the performance of the SWG by reducing the amount of chlorine that binds back to the free sodium in the water. If the SWG is having to work less, then obviously the pH of the pool will rise less slowly as well.

On the otherhand, maybe it's just been too many years since i took chemistry.