gravity32,
The perplexing thing about the idea of speed affecting the operation of a barometric altimeter is this:
The calibration of an altimeter is of the form
z=cT log(Po/P)
where c is a constant, T is the absolute temperature, P is the pressure at altitude z, and Po is the pressure at sea level. The constant c depends on the acceleration of gravity and the molar mass of the air.
There is no v for velocity in the equation.
If there was, then you would expect any supersonic aircraft to use special altimeters.
Now, I am not arguing that the position of the static port is insignificant and that shockwaves and a whole variety of conditions cannot cause errors in the readout.
Those are the fundamentals of barometric altimetry.
The 757 clearly wasn't supersonic and it clearly collided with the Pentagon.
There are many reasons, most of them discussed already, that explain why there is a differential between presssure alt and rad alt without requiring the velocity to be considered.
The g-forces the aircraft experienced when pulling up out of the dive could have introduced even more error.