How are you measuring the pH? Are you making sure there's no stray current? Turn off the electrical devices and then measure the pH, if there's a difference, then there's an issue.
Even if you have a KH of 1 degree, it's going to be about that pH ranges you mention.
Drop checkers stink frankly, stick with the pH/KH/CO2 table, use that to adjust the CO2 relatively.
SLOW Progressive adjustments......... and watch and observe the fish, algae, plants.
Once you get to where things look great, stop, that's your target.
Do this slow and watch after you adjust.