First of all the local Realtor and Real Estate company web sites are the best and most accurate source of listings, and fresh data. The listings are fed to these sites from the MLS and the information is updated a couple of times an hour. The home search button on this blog goes to such a site.
The national web sites are often designed to get home buyers to sign up so that some third party can charge a Realtor a fee to get the buyer's name. The buyer becomes a lead, we call these lead aggregation sites. They use our listings to sell buyers back to us. Other national sites are designed to sell ads to Realtors, or the real estate industry, or to related industries. These sites are able to get some of the listings but not all of them. I personally do not have control over which national web sites pick up my listings, and it is in my seller's best interest to have them on as many sites as possible.
Some of the national web sites require agents to feed data to them manually. Some agents use these sites, others do not. Some of us occasionally forget to update information on the site. When I change the price on one of my listings there are 10 places that I have to change the price. A messy cumbersome, almost unwieldy system. I use a check list for every price change and the process of getting the information out there takes at least an hour. Could I make a mistake and miss a site? Yes, I can and I have. Why are my listings on so many sites? That is what my sellers want, and is what is in their best interest.
As far as I know there isn't any rule that says home search web sites have to have accurate data on them. This statement is going to come as a shock to some but just because information can be found on the Internet doesn't mean that it is true.