It's not so strange. Keep in mind that kh is total carbonate hardness. Bicarb has much lesser effect on ph than carbonate.
Okay here are some actual figures from my logs with co2 injection.
Tap water ph if left to sit ~24 hours: 8.9-9.0
pH in tank with co2 at full bore: 6.6
pH of tank water with no co2 for 8 hours: 7.7 (total co2 equilibrium with air)
kH of tank water: 4 kh (buffered up with bicarb)
gh of tank water: 4-5 gh (mineralized with magnesium and a bit more calcium)
The thing that confuses me is why my pH is 7.7 in the tank with no co2. Could the driftwood really bring down the pH that much?
If I aerate a small bucket with tap water and let it sit for 24 hours there's yet a different story. I don't think my tap water coming straight out is in equilibrium with the air. There's no co2 in the water at all. The pH in an agitated bucket is 8.0.
Why must by water be so wacky?
If your pH creeps up to 8.2 while running CO2, then I would suspect you have something in your tank (limestone rock, shells, etc.) that is dissolving and raising your pH.
Or did you mean it creeps up to 8.2 after you turn off your CO2?
My pH doesn't creep up with co2. It goes down to 6.6 or 6.7.