Interesting! Side topic but that matches my tests as well with api and other test kits.
More on FW
side but (for me) at low nitrate levels around 20ppm Iíve seen varying test results. Each test is consistent but results between test kits can vary by say next colour change.
I did spend a few hundred on an expensive nitrate test kit and even that was saying plus/minus one colour change (for that test +/- 12ppm).