pH is different than acidity.
pH is just a measurement of the activity of the H3O+ ion. Acidity is an intrinsic property of the molecule and has reference the its pKa. In other words one molecule like HCl will become H+ and Cl- virtually 100% in water (pKa -9.3), but something like acetic acid will not. It has a pKa of 4.76 thus at a pH of 4.76 there is an equal amount of acetic acid and acetate (CH3CO2H = CH3CO2-).
pH is also different than alkalinity.
alkalinity is simply a term that denotes a waters capacity to neutralize an acid. Think about this, add a couple tablespoons of baking powder (pH only 8 or so) to a glass of tap water and then see how much of that is needed to neutralize a small amount of soda (you will see very little). Conversely test to see how much ERW at a pH of 11.5 is needed to neutralize the same amount of soda (significantly more). Thus the ERW may have a high pH but very low alkalinity and the EOW may have a low pH, but a very low acidity. (Note that acidity and alkalinity are not opposites, and are both should not be confused with pH).
With thanks to Tyler LeBaron, author