You don't have to know. You can fully remove the few "minor" accidents (that a self driving car shouldn't be doing ever anyway) and the Tesla still comes out worse than a statistical bucket that includes a staggering number of people who are currently driving drunk or high or reading a book

The car cannot be drunk or high. It can't be sleepy. It can't be distracted. It can't be worse at driving than any of the other cars. It can't get road rage. So why is it running into a stationary object at 17mph?

Worse, it's very very easy to take a human that crashes a lot and say "You actually shouldn't be on the road anymore" or at least their insurance becomes expensive. The system in all of these cars is identical. If one is hitting parked objects at 17mph, they would almost all do it.