In this area (Ohio), at least, trace levels of hypochlorite ( bleach) work their way into the storm sewer. If you have a piece of red pH paper, bleach acts like it would on red shirts. It turns it pink. That causes an error in the pH reading. We would double check with a pH meter when something seemed wrong and eventually traced it to hypochlorite. It didn't take much ppm.
The new pH paper might be better than what I used. So please see how your results track now.