Got... When the staffs at particular branch are too slow to put the new price sticker on top of the old one, but in their system the new price is already key in. It happened to me 3 times at the same branch so I know they're slow.

You also will only realise it if you're there buying 1-2 items that you know the price by heart, when you go there you still see the same price, but at counter suda increased.