Any device or kit that is used to measure something can become inaccurate for various reasons. Chemical test kits may be the most prone to becoming inaccurate, since the chemicals used in the test may deteriorate with age or storage conditions, and the techniques for using the test kits may be more important than we assume. This is of no importance if you are measuring stuff just for the fun of doing it, and to get a rough feel for the values of what you are measuring. It is when you are making important decisions about your fertilizing, for example, that you need to be sure the test results you are getting mean what you think they do. For those situations you need to calibrate the test kits before trusting their results.