Well that can be one issue with test kits, repeatability with known strength solutions, in that different manufacturers kits give different readings for the same solution.I’ll prepare some standard solutions and check the accuracy of the API test
The main problem with hobby test kits is ionic interference. Other salts in the tank water can cause test kit false readings. Chloride ions can influence nitrate results, chlorine present in untreated tap water causes a lot of test kits to under read and dechlorinator can cause test kits to over read.
If you insist on wasting time and money testing your water using test kits, then proper "big boy" kits are the way to go. These are generally not affected by other ions.
Anyway you get the idea.