The perception angle is: customers are less unhappy waiting the required 20 minutes for luggage to show up, if they're doing something else during that time.
In this example, the passengers are still waiting while moving (the wait for luggage is 20 minutes either way), they just don't perceive that portion of the waiting as intolerable.
Bringing it home to elevators, the situation is pretty much the same, only in reverse: someone who waits 2 minutes for an elevator and 1 more once aboard it to get to their floor, likely perceives their 3 minute total wait as less tolerable, than if they waited 1 minutes for the elevator plus another 2 once aboard.
In this example, the passengers are still waiting while moving (the wait for luggage is 20 minutes either way), they just don't perceive that portion of the waiting as intolerable.
Bringing it home to elevators, the situation is pretty much the same, only in reverse: someone who waits 2 minutes for an elevator and 1 more once aboard it to get to their floor, likely perceives their 3 minute total wait as less tolerable, than if they waited 1 minutes for the elevator plus another 2 once aboard.