I think I posted about this awhile back on the NACHI forum, I'm going to have to see if I can find my post. From as far as I can tell, they use their business listings. People whose physical address is listed in the city you're searching in will come up first. So it should be similar to going to maps.google.com and typing in the city name and home inspector. If you're not on Google's directory, read the SEO article I wrote (under website optimization on the home page here) and get listed!