CO2 building up in a closed house is actually very common, especially in today's very tight homes, and yes, it can definitely cause issues with keeping PH up in your tank. The glass tops are also notorious for for lowering PH and raising heat!
Aquaman's point regarding what time of day you test is also very valid. That's why I sprung for an electronic monitor. It cost me a little over $100.00, but I now know exactly where my PH is at any time of the day by just glancing up at the monitor. I believe it was money well spent, as PH problems are an indication of deeper troubles in your system.