Usually, but not always, this is a legal den or other such room that the owner is using as a 3rd bedroom. On a few occasions public records are wrong. In most cases the public records are correct and it is the listing agent or owner wishing to enhance (at face value) the market value of the property. Most of the listings on Trulia are funneled directly or indirectly from the listing agent. Every listing agent should review the public records before they list a property, so again, in 9 out of 10 cases it is a known inconsistency. They listing agents often put language in the private remarks (that only agents can see) that say "Buyer to verify all information." Rest assured however, during the escrow process when Title records are pulled, these will say what the property legally is. But this is a good example where every buyer should have their own, and excellent/experienced Realtor representing their interests. A good Buyer's Agent will pick this inconsistency up quickly and appraise you of what are the real facts of the property. Good representation is invaluable, and ironically free for the Buyer (Buyer's Agent's commission is paid out of Listing Agent's proceeds at the end of the sale).