they have SALES and they always have a lot of checkers and then there's the do it yourself lines.
nothing is more irritating than to have to wait, wait, wait in line when you see a dozen open checkouts but no cashiers...like at Walmart.
Fred Meyer and other Kroger owned stores us a technology called QueVision. Basically it records people coming in and out of the store and estimates when they will hit the checkstand. The managers use this information to send more people to the checkouts when they expect a surge.
They also have technology that auto scans your items using lasers as it goes down the belt. I demoed it in one of the Kroger stores near the Cincinnati airport (which is in Kentucky). It is not quite ready for prime time. Works fine for most food products. Not so much for things like clothing.