The cheapo test kits may.....or may not be accurate, only by verifying that they are, can we have any confidence and make decisions based on those readings.
So we calibrate them, then check the test kit against the known standards.
This is true for 10,000$ lab equipment or the 8$ test kit you buy at the local fish shop. The same calibration needs done and once every 1-2 months there after.
JBL, Sera, Lamotte etc, all are not going to be accurate, some might be etc, but you should always have a reference to compare to make sure. It's not their fault, it's just the way testing is done.
You check and confirm if you want to be sure or plan on using the info to make dosing changes or believe that there's a big difference between adding 30ppm or 40ppm of NO3.
regards,
Tom Barr