The POS may have been out of date and the offer has finished but there has been noone available to alter the shelf edge labels. The price you are legally agreeing to is the one that goes through the till, which you should have been clearly able to see at the tillpoint, it is often better to look at the LED display on the till before paying for your purchases. People who work in shops are only human and most stores are understaffed and highly task driven, with a heavy workload, they don't just stand on a till serving customers all day,