The most common error in testing, is caused by not rinsing glassware.
And erroneous results are easily detected by using known standards to compare.

We are dealing in ppm, and even the slightest contamination from a previous sample can cause an incorrect reading.
As a chemist I found unless test tubes are not thoroughly rinsed at least 3 times with DI water between each test, errors are common.
And we used glass in the lab, that is acid washed to remove any trace of residue.
Many of the aquarium test kits use plastic tubes which easily get inundated with nitrate and/or other residue, making them unreliable after only a few uses.
You can prevent this by getting DI water from the grocery store, and thoroughly rinsing 3 times after each test, and once before doing a new test.
Replacing the cheap plastic tubes with a similar glass one also helps, but rinsing those are equally important.
As stated above, the MCL for nitrates in drinking water in the US is 10ppm.
If you are on a public water system, over 10ppm would put the company in noncompliance, and I would suspect sloppy testing methods, before a reading over 10ppm from the water provided in the tap.

If you are on a well, located in a rural area, that may be a different story.