You're trying to do math in a scenario where you have no real numbers. Of course you can pull some stuff out of your butt (+10%, 1 year) to say it's going to save lives but unless you can justify those numbers it's meaningless.
The point is not the exact numbers. It’s to demonstrate that even if we do suffer accidents and deaths from testing self-driving cars on the road, the end result will be many more saved lives (just based on the fact of how many people die from traffic accidents today) Of course this hinges on many other factors to be true, but that’s the case with all kinds of technological advances.