Of course some tests could produce positive results. But not all things tested necessarily produce any positive results. Positive results can be considered as evidence. But negative results are different - because of all the things that can go wrong with the test. At the end of the day there should be a number of tests performed, not just one test by one person. Then analyze the data. If most of the test results are positive you can usually disregard the negative results. They’re outliers. Or you might conclude tests are inconclusive.
The reasons why they were negative might had to do with how the test was performed, was system related or it was something to do with the person who did the test. Of course some things like a placebo should test negative.
Furthermore, there is the issue of HOW GOOD positive results are. This varies from test to test and person to person, system to system. For some people the results may be jaw dropping, for others it might be a big yawn. One should refrain from making blanket statements or drawing too many conclusions from a single test, especially if it’s results are negative.