Did you rinse the probe in deionized or distilled water between readings? Seems like unreasonable readings to me. Unless you are injecting co2 with no buffers, intentionally trying to achieve very low pH or heavily fertilizing with no buffers. Did you test your alkalinity and hardness? I frequently use litmus paper to check the general range. It is cheap and will accurately work from 0 to 14 rounding to the nearest whole number. You could try that to determine if you are in the neighborhood.
I did rinse with distilled water between readings. I do not have CO2 in the 3.8 tank, but have recently added 1 capful of Excel every other day to combat BBA. It is heavily planted.