Why have specials when you dupe customers into paying full price

Why does Food *****s advertise and have specials and then charge you the full price for an item? If it happened once or even twice it could be a mistake but not continually. Just about every time I buy something that is on a special Food *****s they have not charged me the special price but instead the full price. The first couple of times I had already got home when I realised the error. Now every time I buy an item on special I check my slip before I leave. Yesteday I bought their butternut on special for R3.00 each. When i checked my till slip they had weighed the butternut and charged me R11.21. I brought this to their attention. They did not even query or check the price as though they knew they were overcharging and just hoping the customer would not notice the mistake. I am quite prepared to overlook a genuine mistake but mistakes like these don't happen constantly for a period of over a year. Is this a staff problem or is it from the top down?