It’s probably left wide open for automotive interpretation so there can be loopholes, etc. It was probably proposed and passed just to quell the tree-huggers.
If it’s based on averages, Subaru is probably in the worst shape as they don’t have an extremely fuel efficient vehicle like Toyota, Honda or Hyundai does to offset the more inefficient ones. IIRC, most of their cards top out at 28 - 30 highway.