How come some people in the same car accident get a whiplash and others don't?
Good question and one that has been studied by many scientists in the field. It's likely that there are multiple factors involved. Position in the car is one. The force of the impact on the front seat passenger won't be the same as the back seat passenger on the same side. And there is a difference in force on impact from one side of the car to the other.
Speed of crash and direction of the impact can also make a difference. The shear force or mechanical load placed on the neck joint(s) will vary with both of these factors. And the position of the person's head and neck at the time of impact can make a difference. Having the head turned to the left or right increases the risk of whiplash injury.
It's also possible that there are differences in ligament tensile strength from one person to another. Even within an individual's neck, there are differences in ligament strength from top to bottom and front to back.
So, depending on how the impact transfers the load, damage may occur at the weakest ligamentous site. Since everyone has a slightly different ligament configuration, some people may have greater risk of injury than others.