A colleague of mine has on more than one occasion booked several different flights for the same journey, to give himself the flexibility to change his arrangements later. It worked out much cheaper than buying a more expensive ticket that allowed for changes.

For that matter, if you're 90% sure you're going to take the flight (which seems reasonable, considering there's not too many overbooked tickets), you still save money (in expectation) buying the ticket early, since tickets bought far in advance are cheaper.

So maybe it's the better calibrated customers who book flights they might not need.