They didn't have good data to begin with and the engineers complained to Congress that the PA withheld a lot of the documents and info about the building that they requested. Because of these problems, the engineers had to make do with the info that they had. They based their analysis assuming that all the floors above were similiar to the 78th floor, when in fact they were not. From what I can see they also missed the fact that the stairwells were not even in the same places on every floor.
May 5, 2002
The elevator machine room also may have helped contain the upward force of the explosion, protecting Stairway A when it was in the building's center above the 82nd floor. To hold the 600,000-pound weight of the elevator hoists, the beams supporting the 81st floor were twice as big and four times as heavy as those on the 79th floor.
Even the collapse of the two towers did not destroy the massive Otis 339HT machines. The machines were 8 feet tall and 13-feet-by-8-feet wide. Some were found intact in the wreckage at Ground Zero, still bolted to the specially reinforced beams that held them. Investigators at first mistook them for jet engines.
http://www.usatoday.com/news/nation/2002/05/17/stairway.htm
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