Having more than one line materially increases stress for shoppers, because the choice implies they might be on the "wrong" line. So they worry more about how fast they're moving, etc. If you only have one line, you can tell people exactly how long the wait is (because the average transaction time is very stable) and customers don't have to worry about making the right choice.
Having one combined line can increase stress for shoppers too, especially if the line isn't clearly marked by barriers.
What happens is, another customer comes along, sees three cashiers and one queue (in the middle) and decides to join the "empty queue" in front of the cashier at either end.
Of course they know there's only one queue and that they are queue jumping, but they calculate that they can pretend they didn't know.
I assume there's a cultural factor here. I'm in the UK and have seen this many times in one store nearby, but I imagine that in some parts of the world you would be beaten or shot for it. I have never seen anybody complain about it, they just temporarily disperse into three queues (which is awkward because there is not really enough room.)
This is why you really need a new store design. At Best Buy and Whole Foods here, there's a long line that channels into a checkout area that's only reachable via that line. So there's now worry about cutting, and customers naturally form the line as they follow the route into the checkout area.
It's worth noting that WF does use 2-3 lines in a lot of stores where they don't have a long enough space. But then what they do is have a system that signals which line gets to head a register next (and does so in a set rotation), so it's effectively the same as a single long line.
I thought the British were the world champions of queuing, even in situations where there are no affordances, Brits instinctively know how to queue and suffer no jumper.
