I used to do water chemistry testing for large industrial complexes and had many times run across differences in test results that didn't make sense.
Look into age of reageants as a start. Some test kits may sit on shelves for extended periods of time so even though they are "new" to you they may be quite old. I have had this happen on a few ocassions. if you can cross-check with another or a fish friends testing reageants against your own results.
I have also bought reliable testing kits only to have bad reageants in the kits. Don't rule anything out when it comes to quality controls with some companies.
Good Luck, hope you get to the bottom of this. 👍