Really it is you just may not have understood is all, I will try to explain.

Different nutrients are available to the plant at different Ph levels the chart shows that. In turn the plant expels un-used nutrients as well back into the res. That is why the TDS is only accurate when you first mix it. Same theory applies to Ph. The plant expels byproducts that affect the TDS and Ph as well. Depending on the cycle you are in and what nutrients you are using to maintain balance.
Lemon juice for example works fast but it does not hold long the plant drinks up what it wants and expels back the un-used parts it does not need in that cycle. That is what affects your Ph and makes it rise or drop.

The only thing you can do is to keep it in check. Some Ph up last longer then others. I adjust mine twice a day minimum some day's three times. Like when they first enter the flower cycle the Ph move around more then the last 3 weeks of harvest.

So you see the chart and knowing the cycle you can predict how much the Ph will move around on you.

If you want to slow down how much the Ph changes try different Ph up or down products not all of them are the same.