The process of converting a street address into a single point (lat,long) on a map, is called geocoding. Unfortunately this process is often inaccurate and highlights the current problem with traditional addressing.
On our own website and apps, we usually use information from Google Maps, which means that any search on what3words for a traditional address will ask Google where it thinks the place is on the map.
After you've searched for a street address, we recommend checking that it is showing the correct position on the map to ensure you get the right 3 word address: in most cases you will probably want to move the map around to position the pin in the right place.
One of the great benefits of a 3 word address is that it will always point to the same exact point in the world. Street addresses will point to different places on different maps depending on the quality and accuracy of their geocoding database.
For more information on geocoding, see this excellent Guardian article written our former CTO, Gary Gale.