It's possible that age is a hidden variable, certainly, but it's also possible that Saturn, Pontiac, and Mercury are exceptionally low due to selection bias --

It's possible that age is a hidden variable, certainly, but it's also possible that Saturn, Pontiac, and Mercury are exceptionally low due to selection bias -- as you note, they've all been defunct for a decade at least; this means that every car from those brands still being driven has been in use for at least a decade (presumably longer on average). Since collisions and other accidents (statistically) reduce a car's lifespan, the cars from those brands that are still drivable likely have safer-than-average drivers.