That's a misleading phrase. A number of the accidents have occurred because the Google car abruptly came to a stop in a situation where a human driver would not have stopped. Slamming on the brakes is dangerous.
Here is one example. It is hard to be sure exactly what happened, because Google obviously phrases its accident reports to put its cars in as favorable light as possible.
"April 28, 2016: A Google self-driving prototype vehicle travelling westbound in autonomous mode on Nita Avenue in Palo Alto was involved in an accident. The prototype vehicle came to a stop at the intersection of San Antonio Road, then, prior to making a right turn on San Antonio Road, began to gradually advance forward in order to get a better view of traffic approaching from the left on San Antonio Road. When the prototype vehicle stopped in order to yield to traffic approaching from the left on San Antonio Road, a vehicle approaching at approximately 9 mph from behind the prototype collided with the rear bumper of the prototype vehicle."
http://static.googleusercontent.com/media/www.google.com/en/...