That's assuming a 1.5s reaction time. Surely that's quite slow and accounts for the bulk of the difference.
The NHTSA assumes 1.5 seconds because: A typical reaction time to perceive a threat such as a deer or a child running into the road is about 3/4 second, and then you add another 3/4 second to decide to act and move your foot to the brake pedal. I agree it's unintuitively long, but I also know that reaction times have always been slower that I think they are when they've been tested...