IMO, it may also have to do with purchasing behavior. When you think the deal is there for the rest of the month or year, then you are less likely to buy or will wait to think about it or shop around. If you see the price go down and then think the price is going to go up tomorrow, or later in the afternoon, you are more likely to pull the trigger (buy). That is why sellers often run limited time deals, or say this is a today only price.