A very experienced real estate agent once explained this to me, and the more you think about it, the more sense it makes.

When a house first comes on the market, a lot of agents will call people and tell them that the property is a great value, is in good condition in a great location with a great price.  The agent tells you that you better go and take a look at the house before someone else sees it, and sometimes they think they should make an offer before anyone else sees it because it would be such a great house for them.

On the other hand, if a prospect asks how long the house has been on the market and the agent tells you six months, people automatically start thinking that something might be wrong with the property and why has nobody else liked it.  They start to doubt their own logic of liking a property. 

When people do this, they often make a lower offer.  That is why we usually say in real estate that your first offer is usually your best offer because your property has been on the market a short time and people are excited about it. 

We have found that the longer a home stays on the market, the percentage of list price to sales price is lower.  It makes sense for the above reasons.