I'm no expert, but there are a few things I can think of...
The biggest one is that CO2 acidifies water.
Tap water doesn't have to contain more CO2 than atmospheric equilibrium, it could contain much less.
Water with little or no CO2 in it, which is possible for your tap, will go down in pH after absorbing atmospheric CO2.
Water with a lot of CO2 in it, which is also possible for your tap, will go up in pH after eliminating excess CO2.
Last edited by mattinmd; 10-21-2014 at 12:09 AM.
Reason: added non-expert status.