I'd not worry about it, it should be light dependent(pH).

You should measure a mid to low pH right before the lights come on and higher late in the day cycle.

Plants remove the CO2 first.
Then Excel, then KH last.

You have the same amount of buffer(KH), but the acid, the CO2 is removed and then is slowly added back at night when plants do not use it.
So the pH shoots up during the day and returned to lower levels at night.

If you do not use CO2, then do not bother worrying about the pH/KH.
There's little you can do other than add CO2, as that is all that relationship is really used for in our case here(We add CO2 to grow the plants better).


Regards,
Tom Barr