I'm convinced its little more than profiteering. We use several supermarkets, but one in particular is putting many prices up and up, while the others stay static. For a box of coffee (I actually like Nescafe

), one supermarket is 40 Baht more expensive than the one we buy from. The rip-off supermarket used to be by far the cheapest, but hiked the price just before high season a couple of years ago, and its stayed that way. There are other items 'price hiked' for the high season such as insect repellent - my 55 baht product was hiked by 20 Baht - a massive % increase. It's April, so that price is starting to fall again. Even 7-11 is selling my anti-antiperspirant at 50% of the big supermarket cost, and has been for a couple of years.
One supermarket has its offers in little displays in the aisles - yes, there may be the occasional bargain, but most of the items are actually increased in price compared to the day before they take pride of place.
Many items, we buy in bulk and store at home when BOGOF offers are on. If we know we're going to use it, then no problem buying to save.
You have to keep your eyes open, otherwise the fat cat supermarkets will get much fatter.