One thing that is even more surprising to me is:
The Ford Crown Vic is one of the safest rated vehicles while the Mercury Grand Marquis is among the worst rated.
They are basically identical cars.
That might indicate that this is simply accumulated data, and could be skewed by a few particularly bad accidents. Without knowing the particulars, it might also be that the Mercury version has some feature (traction, breaking, etc.) that doesn’t function as intended.
I suspect the age demographics of the respective drivers' samples are somehow related to this discrepancy. Grand Marquis drivers tend to be older, which probably factors into crash survival rates.
It could be that the Crown Vic is safer because other drivers approach a Crown Vic with caution until they are sure that it isn’t a police vehicle.
I would assume that Crown Vics are the cars least likely to be cut-off, brake-checked, tail-gated, or sped past, and that this directly affects the accident rates of this vehicle.