Some common reasons that this happens:
Some stores accidently put the tags on the 2-or-3-day items on Saturday and then have to manually print the extra bucks for them when people say the tags were up. There are some people who go around looking for this because then the items don't show on their receipt when purchased and they can go back and do the same deal on Sunday to meet the limit.
Some employees will set aside the limit of everything for other employees. This is not supposed to be done, but it happens.
Some people buy all the items prior to the sale and then go in and do a return and re-buy on the sale date.
Some managers don't look ahead and order in anticipation of the sale. When that happens, they get the order later in the week and it is usually shorted because other stores ordered ahead of time or the items arrive after the sale is over.
Some extra buck items, especially a CVS-owned brand, are meant to deplete current stock so that no new orders can be placed so what happens to be left at the store is what is available.
Some stores order many of the sale items but the stock goes to a preferred "concept store" that has higher sales volume.
Some stores don't get their stock out, they might have cases of the items in the back.