It's blatant price-gouging. Any stock in the store has already been sold to them at an agreed price. They can set a number and make their set margin.
Updating prices after each delivery might make sense (if their procurement department is absolute dogshit at negotiating contracts), but updating prices throughout the day is just someone trying to see how hard they can push their margins to drain every cent out of their customers.
I mean, the guy who perfected drone strikes on American citizens was never going to suggest that.