OUCH?? Or maybe not?
I may have gone through much the same as you on testing and have a theory that you might think over. Use or ignore is always a question, so feel free to do what seems to fit!
I had an almost impossible time getting anything that made sense with the API nitrate test. I got readings like 80 PPM and above and my fish were happy and breeding, so I felt it was wrong and I got the "shake the bottle " answers and went as far as building a bottle shaker on my reciprocating saw so that I KNEW it was shaken. Somewhere along the line, I got the idea that having used both bottles but one not fully shaken, I might have used the "good stuff" out of one bottle and not the other, so the answers were going to always be wrong, so I bought a new set, shook totally and still got wrong answers!
That is when I went for the slightly more expensive Salifert test for nitrate and find it may be more expensive but it actually tells me something useful!
I was finally able to test and do a 50 % water change and see the reading go down 50% ! I'm glad to pay a nickel extra if I don't have to beat the bottle to death each time? I can actually see and read the difference in the pink colors. Wow, what a relief.