City tap water is often treated w Chloramine. Even after letting it sit for a couple days, it will try to buffer the ph to a neutral, or near neutral state- usually to the slightly acidic side. I had this problem using my tap water in hydro. After a point, the ph adjusting solution will exhaust the buffering qualities of the chloramine, and the ph will stabilize.
I've mentioned this on previous posts, and dawn some fire and ridicule from the "pros", who claim that this simply does not happen. I have several friends who work for the city, w/ a couple who work for the water dep., who have confirmed, beyond a doubt, that yes, this DOES occur- by design. The chloramine is added to prevent fluctuation swings in ph.