That's a difficult answer to give.. I've got 2 permanent pH meters Hanna and Milwaukee, assumed to be the beter part of the stuff out there and a few pocket versions from i forgot what brands.. Never mind, not 1 is equal to the other and all get calibrated in the same fluid.. Not huge differences but still they all have a few "0.somewhat" tolerancy compared with eachother. So beats me which one is most accurate.. Actualy it also realy doesn't worry me a bit, over the years i found out that pH aint that important so that you would need a spot on accurate measurment.
Next to that, the probes is filled with a pottasium chloride solution.. This solution can or likely will degrade and change it's propperties over time and should be refreshed.
This slow degration process likely is the cause of small changing measurments. Some Sensors have a refill option others do not. The ones that do not, should be replaced if it doesn't calibrate correctly anymore. Usualy by marketing standard once a year is adviced.. But cheaper to do it only if calibration becomes an issue.
There are also refillable pocket versions.. The ones that don't are ready for the trashbin once reading gets off and can't be calibrated anymore.. Funny is reading the manual of these non refillable pocket versions do not state 1 year life time. Why that is?? Maybe manufacturers assume that measurment with a pocket device doesn't need to that spot on accurate..
Anyway.. IMHO in our hobby, if you run Co2 on the brink of destruction, than very accurate measurment could be an issue.. Do you stay in the safe range than a few 10th of discrepancy up or down isn't something to worry much about. Bottom line a bromothymol blue test is actualy all you need to get an relative good indication.