His water is already somewhat hard. Your water's pH has a near neutral pH to start ... Remember what I said about your well water being of better quality than my municipal sources? You're lucky in that sense.
His water is at the higher end of accepted alkalinity. For example, if he were to pH down his water and set it aside for a couple of days and then come back to it and retest the pH ... His water will have likely risen well above where it read when he initially took the pH reading. This process describes the alkalinity of his water.
Your water comes out of the ground just about where "natural pure water" pH's at. This means the ppm in your water is not effecting its pH. Also, promix isn't soil. It's peat which has a tendency to drift downward no mater how well buffered it is. In my eyes, it's not a direct comparison between what you're doing and what his situation is. I'm also inclined to believe that if your water started with a higher pH and you were to use pH down to adjust your nute water to 6.2 lets say ... that the phosphoric acid "usually" used in pH down would eventually cause your promix to drift downward out of range over time. It's a strong acid and will continue to drop the pH of your water over the course of a few days if you were to try the same experiment I suggest to @Dothraki?
Same experiment yet different results? Water chemistry isn't the same which is what much of this thread has been about.