By virtue of the typical working hours of engineers alone, they are unlikely to drive in the same conditions, roads, and traffic as normal drivers. Their operating hours skew heavily towards driving in city traffic during non rush hours. They skew heavily away from driving at night, during rush hour, during lunch times, and during extreme weather events.
I would also feel comfortable assuming that even if they put an effort into driving at similar times as other people, they would still skew away from scenarios that engineers find boring. Good luck getting them to drive in 2mph stop and go on 880 or 101 during rush hour.
Seeing them on the road commonly is not a guarantee that their driving patterns are representative. Their driving patterns are deliberately designed to not be representative...not because it helps their metrics, but because it is the only feasible path for development. Back when the first DARPA challenge was around, the only objective was to make it from point A to point B in an empty, flat, and mostly obstructionless desert (and everybody failed). By the time of the last challenge, they had to navigate a suburban-ish area devoid of people but populated with demarcated roads and carefully placed obstructions. Now they are on relatively slow city streets with actual people but controlled times/conditions.
So this phenomenon is obviously not new...SDC engineers have known this from the beginning. But now we have a new class of SDC boosters that don't actively involve themselves with development and don't seem to realize that their metrics are representative of the current state of SDCs and not a guarantee that can reliably extrapolate.