It used to be that we got all Hetch Hetchy water in the south bay. Since the California drought hit, they have started getting creative with where they pull water from. Some homebrewers that I talk to say that the water chemistry changes periodically, even over the course of a week. It matters for homebrewing because things like the calcium salts present in the water can change the pH of the wort. So I wouldn't assume that just because you tested the water once, that you know what it will always be like.