Seems indicative of a supremely dark sense of humor (he wanted to make sure he got credited for the flight segment) and/or meticulous preparation (putting an FF# in would possibly invite less suspicion at the gate/checkin).

http://www.nytimes.com/2001/09/15/in...al/15GERM.html

Here are the closing paragraphs of this article:

After the gap in the first months of this year, investigators pick up the trail of Mr. Atta on May 2, when he entered the United States using a visa issued in Hamburg. Within two weeks, he had rented the apartment in Hollywood and begun sharpening his flying skills on a single-engine that he rented for $88 an hour in Palm Beach County.

He appeared with an unidentified friend and said he wanted to increase his flying hours. He apparently convinced the flight instructors that he was already a certified commercial pilot with 300 hours of flying time.

"He was not working toward another license," said Andrew Law, a flight instructor at the school. "He was just practicing, showing his friend the airplane and what to expect."

The purpose of this practice became clear within three months. According to an F.B.I. document obtained by the magazine Der Spiegel and shown to The New York Times, Mr. Atta emerged on Aug. 28 to book a reservation for a one-way ticket for Sept. 11 on American Airlines Flight 11. He bought his ticket on the American Airlines Web site and used his mileage card: AAdvantage Profile #6H26L04, which had been established only three days before, on Aug. 25. To pay, he used a Visa card.

He also paid for the ticket of Abdulrahman Alomari, who traveled on the same flight. Six of the suspected hijackers used the same contact telephone number as Mr. Atta, (915) 815- 3004, suggesting that the reclusive engineering student had taken on a central role in organizing an elaborate and painstakingly prepared plot.

Mr. Atta traveled to Boston on a commuter airline from Portland, Me., with Mr. Alomari. Mr. Alomari was assigned seat 3C; Mr. Atta, seat 8D.

One of Mr. Atta's bags did not make Flight 11. It contained airline uniforms, a video on commercial aircraft and, the F.B.I. says, a suicide note.