There were 8 real accidents on the road. In all 8 of those real accidents the airbags were deployed. Safety drivers were in some of those cars and took control of the vehicle at some point. When the safety drivers in the car took control; they performed evasive maneuvers that reduced the potential outcome from "most severe or potentially severe" to something less severe.
When the engineers reviewed all of these 8 crashes they played them back watching what the humans did, then they put all of the constraints into their simulation and let the AI take over. When they say "Three of these crashes occurred in real life and five only in simulation" that means that 3 severe crashes happened in real life and the AI would have cause 5 more "severe" crashes had the humans not taken control.
When the engineers reviewed all of these 8 crashes they played them back watching what the humans did, then they put all of the constraints into their simulation and let the AI take over. When they say "Three of these crashes occurred in real life and five only in simulation" that means that 3 severe crashes happened in real life and the AI would have cause 5 more "severe" crashes had the humans not taken control.