Logically, there was a 20th hijacker, it probably wasn't this guy, but someone gamed out taking over airplanes with boxcutters and figured out that you needed 5 guys to make it work.
So someone missed the plane, chickened out, didn't feel good and took a sick day, or maybe never made it into the country, but they wouldn't use 5 man crews if four man crews worked just as well.

My opinion, it was a last minute no-show, because they waited on that plane and let it fly way past the logical takeover point, probably debating whether to go ahead with it or not, and when they went ahead with the plan, it didn't work, they were a man shy, what more evidence that 4 man crews don't work could you ask for.

Which also tells you that they did some awfully sophisticated planning and testing, because they knew that a 5 man crew was required, that's not exactly an obvious number, and they apparently knew that 4 men wouldn't work, but that 5 would.
Because it's not the kind of mission you send extra men on, just in case.

Which means that somewhere there are a bunch of people who played the parts of passengers and flight crew, maybe they are all dead, it's easier to keep the secret that way

__________________
“The thought of his being president sends a cold chill down my spine.”

Senator Thad Cochran, Mississippi (R)
on McCain

“I decided I didn’t want this guy anywhere near a trigger.”
Senator Pete Domenici, New Mexico (R)
on McCain

“My anger did not help my campaign ... People don’t like angry candidates very much.”

McCain on McCain