The reading you get depends on the intensity of the lines on the test strip — and those lines change over time as the strip dries out. About 1 hour after you collect your saliva sample, the test begins to dry, which alters the line intensity and produces a different (inaccurate) reading.
This is why we recommend scanning your test at 20 minutes after collection, and no later than 1 hour after collection. Scanning outside that window won't reflect your actual cortisol level — it reflects how much the strip has dried.