The subjectivity with the API Nitrate test kit is with the interpretation of results. Some of the values (40, 80, 160 ppm) are very similar and can be difficult to tell apart. If you keep up with water changes as BBradbury mentioned, the nitrate levels will stay low and be much easier to read.
Being consistent with how you perform the test is important as well. This includes the manner at which you shake the reagents and test tubes and the times for each step.