The test procedure was done in accordance with the SAE (J 575) testing protocol designed for rear lighting on passenger vehicles and performed numerous times to ensure repeatability and accuracy.
This what you're showing is a graph of the rise time of the light from "off" to "lit". That's not part of the compliance testing requirement. It appears to use 80cd as the "DOT requirement" line. 80cd is the minimum required intensity for a stop lamp (or a red rear turn signal) at the H/V test point. There are numerous other test points not covered by this graph, and intensity ratio requirements, and many others you don't appear to be aware of or care about.