An orderly queue of one

[ewx]

I often go home from work via Sainsbury's to get some shopping. I've noticed the following things about the queues.

There are a dozen or so traditional tills, with one operator and a conveyor belt. Not all are always in use, but they generally have about equal queue lengths, usually between 2 and 5 depending how busy the place is.

There is also, at one end, a cluster of about half a dozen smaller tills, with just a shelf for your basket, and a single queue for the whole cluster. It's rare that all the tills are manned but I don't think I've ever seen them with less than three in use, usually more.

The cluster queue is usually the same length, to within one or two, of the main queues, despite the fact that it drains three to six times as fast. (No prizes for guessing which queue I choose to join.)

(A trolley would would be a bit awkward for the clustered tills but (nearly?) all the shoppers in this supermarket use baskets.)