Former Sen. Bob Kerrey and former Rep. Lee Hamilton, both Democrats, determined that their time was more valuable than the president's. According to The Washington Post:

Two of the Democratic commissioners left the session about an hour early. Vice Chairman Lee H. Hamilton was scheduled to introduce the Canadian prime minister at a luncheon, and former Nebraska senator Bob Kerrey left to meet with Sen. Pete V. Domenici (R-N.M.) on funding issues related to New School University, where Kerrey serves as president.

The 9/11 commission insisted that it was so important that National Security Adviser Condoleezza Rice had to testify in public before the commission. Originally, President Bush and Vice President Cheney were going to meet only with the panel's chairman and vice-chairman -- something that was deemed unacceptable.

After all their complaining, Hamilton and Kerrey couldn't reschedule their appointments. Obviously, whatever the president had to say wasn't important enough for them to stay around to hear the entire thing.

The Post's report didn't mention the early exit until the very last paragraph. That's bad, but it's better than the "paper of record." The New York Times doesn't mention it at all.