My Direct TV Program Guide shows the 9/11 mini-series to be a total of six hours in length - (Part 1) - three hours on Sunday night and (Part 2) - three hours on Monday night. My live Tivo guide which is more current shows three hours on Sunday and two hours on Monday! It is presently scheduled to be five hours in length. ABC may have originally planned on six hours when the earlier guide went to press, but were not able to line up sponsors for the final hour and edited it back to five.
Another possibility is that the program was edited to take out portions that may have contrary to the story the right-wing pre-screeners wanted told. However, at this point we are left without knowing the reason or reasons for the change. A interesting third possibility is below the fold.