Yes, this is as the buffers cancel the acids/alkalines and there is still imbalance and I think I read somewhere it is because the water is absorbing ? CO2 ? from the atmosphere and that caused the PH to change. I like at least a 24 hours sit before checking PH, but never had it continue to change after the first 24 hours....but I use about 5 gals a day total, so I just let one bucket a day sit out.