You make the analogy of queuing, which is itself not smooth. Some customers have more items or will take longer for other reasons, so much like progress bars, queues do not move at a steady rate. And the time estimates given for places like that are just as inaccurate as any a computer progress bar unless the service sets a hard limit on how long each customer can take, which I think might be a thing in a few places. Not stores, but certain services, usually ones that require an appointment.