An engaging and helpful discussion with an employee working in the crash test department.
He spoke about ISO fix and also recommended this type of car seat. It works with two hooks that clips on the back of a metal bar found in modern/ recent cars. The pressure/ stress acts differently. When the parent brakes the movement of the chair does not move with the force of the car, it has a different type of impact as it is fixed on the metal bar at the back making it much safer for the child. The ISO fix has a hinge point which is attached to the bar.
Car seats which rely on the seat belt has a totally different impact when in an accident or type of force. When force is applied from the car the car seat reacts much differently to the ISO fix. The seat belt reacts much later as it locks with a certain amount of force applied giving away some time with the seat belt. This allows the car seat to move with the force of the car compared to ISO fix having a different reaction to the force. A confirmation is the pressure points is based where the seat belt is in contact with the chair.