I noticed they're "grabby" but never put 2 and 2 together. I'm guessing the ACC kicks in the ready alert braking which i think basically applies enough pressure to get the "slack" out of the brake system. That way there is NO free play so that the brakes respond a fraction of a second sooner.
(just a guess but perhaps the ACC needs to get the variable out of the way that the "slack" provides. If it's calculating braking distance to maintain proper distances maybe they didn't want to put in yet another variable into the equation?)