It all has to do with what's going on around us. The speed of the vehicles, the type of vehicle, the safety factors, seatbelts, where you are in the car, what was the force of the impact, etc; Every accident has different factors playing into them in order to lead to said injuries
Angle of attack ,velocity ,seatbelt health, airbags, cumbustion of the car ,car type, saftey features ,location of collision aka being hit in the drivers side door is worse then being hit in the trunk