The recorded "altitude" of Dulles was 40 feet. Using standard atmosphere this calculates to 29.88 inch Hg. Then using known actual atmosphere this converts to 312.8 feet, which seems about right for Dulles.
Similarly at the end of the flight the raw altitude, -99 feet, calculates to 180 feet. The radio altitude is about 56 feet thus the difference is 124 feet.
Have not looked at the DFR data (and dont have time to do so) but wanted to mention another factor that 'may' explain some of the apparent errorr:
Depending on sampling rate/ dfr frame rate position it is possible that the the last RA altitude was at a later point than the barometric altitude.
Dont know if the vertical rate was high enough for this to matter but is another point to consider.
Pardon in advance if this has already been accounted for.
BTW: I agree that trying to convince the "true believers" is futile but also know that lack of response can lead semi rational people to the wrong conclusions so this is not wasted effort.