Same happens at tesco, it takes a time to rectify price changes instantly off the system so the tills get the instant change and the shelves get them when they can, Tesco have a policy to charge the lower price, its up to the customer to question the price first.
It can work in your favour, I bought four packs of biscuits in a buy one get one free offer where the till and shelf price were different and got the biscuits for nothing and actually made 2p! yep i was given 2p out the till without actually paying for the biscuits, ridiculous situation or what? Guy did it with buying all the bannanas up years ago remember?
There was a system shown years ago on Tomorrows World(remember that program?) which used a device connected through the ballasts of the flourescent lighting making it flash rapidly(faster than the eye could see)which communicated with intelligent labels on the shelves and updated them at the same time as the tills, wonder why it didnt take off maybe technical issues or cost? with wireless and RFID tagging etc it cant be long till LCD displayed prices come incorporated in item labels?