Engen/Woolworths Bakenkop Service Station- Inflating prices of goods.

Engen/Woolworths Bakenkop Service Station (Centurion): The prices for (some) goods, not all, on the shelf is marked lower as the price scanned at the paypoint in the Engen/Woolworths shop in Bakenkop. I have been complaining to them for about 2 years now, they simply do not fix their prices in order for the shelf price and till price to match. I wonder how much money they make by doing this? If a product is priced on the shelf, I do not have to pay a cent more when scanned at the till. Every single time I have this problem, and it's always the same explanation. Surely its not rocket science??