There are a variety of reasons this could potentially be. If the the property is a foreclosure the selling price may be listed as what the bank needed to bring to the table to basically buy back the property from the home owner.
Another explaination may be this is what is called a non-arms length transaction. For example if Mom and Dad want to sell thier condo to a son or daughter, there is no reason why they would have to sell at the current market rate.
Taking a look at the tax records usually is a good way to determine the type of sale.
Hope this helps!
Dave Tap Tapper