Japanese car (JC) is relatively soft. When doing the standard crush test, the soft engine compartment (all the room under hood) can absorb a lot energy and thus driver/passenerger be protected.

American car (AC) is relatively stiff. When doing crush test, the hard engine compartment (all the room under hood) can absorb less energy and thus driver/passenerger be protected not as well as JC. That is one of the reason that AC doing not as well as JC in the standard crash test.

However, in the real world, pelple don't drive their cars into a cement wall (like in the standard test), the most common happened legthal accident is head-on (head to head) collision. In situation like this, the hard one (AC) will crash the soft one (JC) and use it to absorb the energy. Driver in the AC will have bigger chance of survial, and normally JC will have bigger damage. Check the insurance rate (for colision protection) for AC and JC and you will see JC has relative higher rate.

Since 90's, the difference between two groups of cars are getting smaller and smaller.