The lower the buffering level the less stable the pH. When not buffered gas exchange alone can change pH. So just the act of taking the sample could greatly change the measured pH.
The standard AM/PM cycle of pH, low in the AM, higher in the PM, still depends on some buffering to appear. How much swing seen in this cycle indicate how effective the buffer is. For example a pond can be buffered with say oyster shell.If acid is added the oyster shell will indeed use up the positive hydrogen ions produced by the acid, but it will do so rather slowly. Where as something like baking soda dissolves in the water and sucks out positive hydrogen ions very fast and the AM/PM swings will be less. So while many things are buffers, some are better than others.
Water can be buffered to pretty much any pH. Adding a lot of acid would want to stay at a low pH because the acid released a bunch of positive hydrogen ions which is what pH is measuring. Or water can be buffered to a higher pH by adding something like baking soda, measured by KH. pH wants to stay high because positive hydrogen ions react with the buffer to form another substance, so there are fewer positive hydrogen ions to measure and pH is higher.
When water isn't buffered, either low or high, it means there aren't a lot of positive hydrogen ions to measure, which would mean high pH, but there also isn't anything sucking up what positive hydrogen ions there are, so water can be low pH, at almost the same time. Swings are at the whim of simple gas exchange. So to me pH is a pretty worthless test on its own.
