Oh dear, where to begin...
Instruments such as pH meters/controllers, test kits, thermometers and any other measuring instruments (even teaspoons and tablespoons), have a built-in error. The better the instrument, the more accurate, but there is ALWAYS an error. The Milwaukee SM122 controller has an error range, stated as an accuracy of +/- 0.2 pH units. The Pinpoint Controller, too, has an error range, though I can't find it on their website. A pH pen will most likely have an accuracy of +/- O.2 or greater. Needless to say, we can never measure pH EXACTLY with our measuring instruments, but we get close enough and close enough is good enough for our purposes.
Knowing this, we can now see that comparing pH reading results, from different pH meters and test kits, and wondering which one is correct is useless and causes much needless consternation. If both meters have an accuracy of only +/- 0.2, then you could have two results that are as much as 0.4 pH units apart. This isnít wrong or bad, itís just the accuracy of our instruments and test kits.
There are other variables that can ADD to the error already present in an instrument. One is the condition of the electrode. How old is it? When was the last time a calibration was checked and how fast did it get to pH 4 and 7? Is the electrode tip clean and free of algae or debris? Each of these variables can compound error upon error, so that accuracy is further diminished. Itís no wonder many plant aquarist, including Mr. Amano, donít use pH controllers at all, relying solely on a bubble rate and observation.
Accuracy vs. Consistency
Given all these variables that can affect pH readings, including the ďfreshnessĒ of calibration solutions (they expire), we can see that the CO2/KH/PH Chart becomes more of a guide and explains why some people, including myself, can maintain a ďtheoreticalĒ CO2 concentration of greater than 30 ppm with no apparent ill effects. Accuracy does not mean exactness, rather, itís a narrow range of error that comes with all measuring instruments and implements. So if your equipment is in good working condition and you are getting CONSISTENT readings from your pH controller, then I would trust the results, but only down to +/- 0.1 or +/- 0.2 pH units, For our purposes, consistency matters more than accuracy.
It appears that you are getting consistent results from your pH controller. The readings arenít fluctuating or drifting greatly and it sounds like you have no trouble in calibrating it. I would say your controller is functioning as it should given the accuracy of the instrument. Check your calibration solutions for an expiration date and get new ones if necessary. Used solutions should be dumped not saved as this can also compound more error into the accuracy of the controller.
I hopes this helps you and others in the same quandary. Just know that no two test results will be exactly the same, except perhaps by chance.