Why an idle elevator changes floors
An elevator group is not just a set of cars answering calls one at a time. Its controller also has to decide where unused cars should wait, because the next passenger could appear on any floor. In many buildings, the lobby is the most likely starting point for a trip: people enter there and head upward. Keeping an available car nearby can therefore shorten the wait when several passengers arrive close together.
When a car finishes a trip and has no assigned call, the controller may send it back down rather than leave it wherever its last passenger got out. That repositioning uses time and electricity, but it can be worthwhile if it puts the car where demand is most likely to appear.
A prediction, not a guarantee
The lobby is especially useful as a staging point during periods when people are arriving or leaving in clusters. But sending every idle car there would create its own problem: cars could bunch up at the bottom while passengers upstairs wait. Group controllers balance likely demand across floors, assigning cars to calls and sometimes keeping them in different areas instead of returning them all to the lobby.
That is why a car may seem to pause or take a longer route before reaching you. It may be completing an existing assignment or moving to a position the controller expects will help the next call. The goal is not always to make one ride as short as possible; it is to reduce waiting across many passengers. A car sitting at the lobby can look idle, but it may be positioned there for the next rush.
