Aside from other factors, one that surely worked to the advantage of the 9/11 hijackers was that each of the four flights they caught on that fateful day was carrying significantly less than a full passenger load. Again we have what appears to be little short of a miraculous concatenation of timing and other circumstances:

Since the four planes, like the World Trade Center, were mercifully not full to capacity, casualties ultimately proved less than they might otherwise have. On the other hand, the hijackers also had fewer passengers to overpower or keep under control, and there was less passenger weight to use up fuel. Magically, the flight that carried four hijackers rather than five — United Airlines flight 93