The difference between the "wreckage" crashes shown in the video and the "pentagon" crash is that the wreckage crashes probably took place around 200 mph or less, while the pilots were trying to land or take off (low angle of incidence). The 757 that crashed into the pentagon was flying at least 500 mph, while trying to crash straight into the pentagon (high angle of incidence). The plane at 500 mph carries 625% of the kinetic energy that a 200 mph crash would. Its hard to imagine, but the plane parts simply obliterate themselves into tiny shrapnel (no large pieces), much as the front of the fuselages do in the "wreckage" crashes (it even shows this in the video).
Here's a government test of a F4 phantom vs. a heavily reinforced concrete wall (I'm sure the pentagon is made of pretty strong reinforced concrete).
http://www.nod.se/sandbox/2004-09-20/concreteplane.wmv
Or maybe I'm under the influence of the brain control machine..
