Sometimes sellers believe it doesn't really matter how many days their property is on the market. Particularly if they have no pressing need to sell, they figure, "We'll just leave it on the market and if a buyer wants it they'll pay what we're asking".  As the Days on Market (DOM) count rises, their chances of selling at their asking price decrease. 

Although there is not linear correlation (the black line is the trend line), the graph below illustrates how as days on market increase, the ratio of sale price to original asking price decreases.