Most supermarkets promotions are nowt but a con, especially the price match bollocks. Price match means we will match the price, it does not mean it will be cheaper than when the product was previously sold in the shop, in fact it can be a higher price than previously sold in the same store.
Always check the weight of what you are buying, again people will pick up promotional items without checking the weight and again could actually be paying more pound for pound. And always check your receipt, sometimes products are dumped through on the wrong code, so that lettuce you bought for a quid, just actually cost you £4 because you didn't check.
The weight shrinkage is probably the biggest thing used by manufacturers at the mo to cut costs, or specific ingredients, where a product previously had 20% of a specific ingredient it now only has 10% and had more **** chucked in it instead.