Ummm, sorry, yes they do. Over a course of a year I was surprised that my till bill was consistently higher than I thought it had been. Then over a month or so I added up as I went through the shop, and rounded UP as I went, only to find that my bill was still higher than I'd counted. That day I went back around the shop and found dozens of products charged higher at the till than listed on the shelf. I watched this for weeks before confronting the manager the say I went in solely to buy a frying pan (£20) which I was charged £25 for at the toll. She claimed it was a simple mistake, and they're likely in such a large shop. I pointed out that every single mistake was I Tesco's favour, but that cut no ice. Have since concluded that this was a matter of policy and have never been into a Tescos since.
Other cons the big supermarkets have, like counterintuitively charging more than double for 100g of tea, compared with 50g of tea, or offering a meal deal, eg curry, and planting a food in the middle of the display that isn't in the deal, so you don't qualify at check out.
They rely on the fact that most people either can't add up, or are simply too much in a rush, or worse, they trust a British institution to be honest in their dealings. Yup - you get big by bring clever at ripping people off!