Final answer:
Lester missed his flight because of an unexpected last-minute change to the flight schedule.
Step-by-step explanation:
Lester gaped in disbelief when he saw that there had been a last-minute change to the schedule, and he had missed the flight. The correct interpretation of this sentence is that Lester missed his flight due to a schedule change. This implies that the change was unexpected and Lester was not aware of it in advance, leading to his shock and the unfortunate situation of missing his flight.