Every time I go to my nearest bus stop I wait and wait and then three or four arrive at the same time.
Buses may be leaving the depot at regular intervals but people arrive at bus stops in drips and drabs. At some point a sudden burst of potential travellers will turn up, and when the bus arrives it will have to stop for longer than normal. That gives the bus behind time to close the gap between them. The second bus arrives at the stop soon after the previous bus left, when few people are waiting. So the trailing bus gains still more on the bus in front. This is a cumulative process. The bus behind will always travel faster than the one in front. Which means that, eventually it will catch up. The two buses will end up travelling together allowing a third bus behind them to catch up in turn.