Honestly, I used to wonder the same thing. I’d test my pool every week, thinking I was doing enough, but the levels still moved around more than I liked especially on those really hot days.
What worked better for me was testing every two or three days instead. It’s not as annoying as it sounds, and catching little changes early made a big difference. I also stopped adding chlorine during the day, now I toss it in at night so the sun doesn’t zap it right away.
Keeping an eye on alkalinity helped too. I didn’t realize how much it affects pH until I started paying more attention to it. Once I got that dialled in, the pH didn’t swing as much, and chlorine seemed to last longer.
It’s not a perfect science, but with a bit of a routine and some tweaking, it gets a lot easier to keep things balanced.