• if the floor plan isn't what they're looking for
   
  
• if the finishes and materials aren't what they're looking for
   
  
• if the location isn't what they're looking for

Let's imagine a house with a market value of $300K.

• Initially listed at $309K, the seller was hoping a buyer would come along to negotiate a deal of $295K or $300K.  It didn't happen.
   
  
• The seller then reduces the house t o$295K.  Given assumed negotiating room, the house might now be purchased for $10K - $15K below market value.  But if there isn't a buyer in the market looking for the seller's combination of floor plan, finishes and location (among other things), the house still might not sell.
   
  
• In some ways, the margin would have to be even greater to get one of the buyers currently in the market to jump.  Perhaps a list price of $280K with an assumed potential purchase price of $270K-$275K might make a buyer look past what they really want as to floor plan, finishes or location.

As a seller, listen carefully to buyer feedback.  If you have a buyer who likes the floor plan, finishes and location, don't let them disappear based on price -- encourage them to MAKE AN OFFER.  But if most buyers who come to see your home have feedback related to floor plan, finishes or location, don't assume that lowering the price by $10K or $15K is necessarily going to reel those would-be buyers back in.