It is possible that some tap waters have low pH. That is most likely for sources that have very little alkalinity in their water. With that said, tap water pH has very little to do with subsequent brewing water chemistry. The most important factors are the actual ionic content and the critical ions that affect mashing pH: Ca, Mg, and HCO3. If those ion concentrations accurately represent what is in the water, then a brewing chemistry program has a decent chance of predicting pH for the user.
In the case of a difference between a Ward report and tap water, tap water is more likely to contain more dissolved CO2 since it was under pressure in the distribution piping system. If you leave a sample out and its pH rises over several hours, it is a likely sign that the water has excess CO2. If the pH falls over time, then its a sign that the water has little alkalinity and little dissolved CO2.
I guess it is possible that Ward vacuum degasses the samples they receive, but I don't know. I don't recall if degassing is a required protocol for testing.