Bunching Buses - Perception Or Pattern?

Perception or pattern?

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You’re standing at a city bus stop, waiting for a bus to arrive. You wait and you wait. There’s supposed to be a bus every 10 minutes, but you haven’t seen one for at least 20 minutes. Finally, a bus arrives. It’s full of people. Just as you try to squeeze yourself in, you spot another bus coming down the street, and another one behind it!

What’s going on? Why do buses always seem to come in bunches instead of at regular intervals?