I recently had a very similar question about my well water pH. I was puzzled because my well water was coming out of the tap at 6.6, and after a week of sitting in a rain barrel it tested around 7.5. This is the answer I got from one of the experts over at TFK.com.
"It would seem that you have a considerable amount of dissolved carbon dioxide (CO2) in your well water. This often occurs in tap water too, which is why it is recommended to let tap water sit for 24 hours before testing the pH, as this gives the CO2 the time to dissipate from the water. CO2 lowers pH because it is adding acid, and when it dissipates from the water the pH rises. This is the same in reverse as when planted tank aquarists add diffused CO2 to their tanks, it lowers the pH."
"Sometimes objects in the tank that are calcareous (rock or gravel made from calcareous rock, coral, dolomite, marble, lava, etc) are the culprits but in this case you have tested the water on its own, so I suspect the CO2 is the answer."