The map function only loads a certain variable (x) amount of properties. Trulia also likes to show what homes have sold in the area recently (red homes). However, when the map is zoomed out all the way, the green houses only show up. The reason is because people browse Trulia primarily for listings currently for sale. People also use Trulia as a research tool to see how much their home could be worth.
As the zoom function moves inward, more properties populate the map for that particular geographical region. Meaning, if the map only loads (x) 250 properties at a time, you'll only see that many homes for sale on a large geographical region. The same amount of homes is displayed within a (let's say) 0.3 mile radius. However, if 250 homes are not for sale within that radius, Trulia defaults to allow "Sold" (red) and "Off Market" (grey) listings to populate the map as well.
The strategy is two-part. 1) It may generate a "eureka" moment for home seekers to use the tool for not only looking for houses for sale, but to also use Trulia for comparable sales. 2) As an SEO strategy for the website to always have random content which changes.
It's exactly the formula I use on my personal site when building the code structure to support what people are searching for. The "load" or "weight" for a webpage to carry more than (x) amount of data would produce a "crash" on most browsers. Therefore, limiting the searches to a certain amount of data ensures a faster search experience for web users.
Simply put, if you want the most accurate data, use a local agent's site. Preferably an agent who is aware of these issues and can adjust his/her website accordingly; thus giving you the latest and most accurate listing data.