It's not that. The water may have quite a bit of buffering capacity. The effect of brewing water on mash pH (its buffering) is the same as that of any other mash component. It depends on the alkalinity of that component relative to its intrinsic pH (the pH at which it comes to you) at mash pH (buffering). Here we are speaking of alkalinity generally. It is the amount of acid required to move a unit amount of a mash component from its intrinsic pH to an arbitrary pH. In predicting mash pH that pH is, of course, the pH we want for the mash. When we look at a water report, however, the implied end pH is 4.5 (for the ISO standard definition of water alkalinity). This is lower than mash pH so reported water alkalinity (to pH 4.5) is higher than the water's alkalinity in a mash (to pH 5.4 or so) but not by much as bicarbonate's (which is the major cause of water alkalinity) titration curve starts to level off as mash pH is approached. To really appreciate what I am talking about here you would have to look at the bicarbonate titration curve and calculate some alkalinities to various mash pH's assuming various water intrinsic pH's. You will find that the actual alkalinity presented to a mash does indeed depend on both mash pH and water pH but will quickly discover that mash pH is about 89% of reported pH in most cases. That's because it only takes about 11% more acid to move water to pH 4.5 than it does to move it to 5.4 (because of the flatness of the titration curve in that region). Thus we often say that the effect of water's alkalinity on mash pH does not depend on the water's pH. This is true but only approximately so.
