I bet it works better for Costco because they don't stock any items with weights low enough not to be registered by the scale.

Also, the last time I went to my local Costco, you were no longer permitted to check yourself out at the self-checkouts. They didn't remove them, but they had started using them as cashier-staffed checkouts.

Mine still lets you scan your own items. I bet they only have employees scan items at stores with higher loss rates.