The difference is whether the collision is allowed to make the wall move. A 100-mph car won't be halted by a physical brick wall, but will just bust right throu

The difference is whether the collision is allowed to make the wall move. A 100-mph car won't be halted by a physical brick wall, but will just bust right through it. Afterward, both the car and wall remnants will be moving at 50 mph (minus however much energy it takes to tear the section of wall from the ground). In that case, the change in kinetic energy will be not much more than in the car/car case. For a truly immovable wall, the ground has to exert a lot of force to keep it in place, which goes into the car, causing the entire discrepancy.