One critical element of a gps navigation system is the the routing engine that is inbuilt into a machine. The routing engine is software that figures out which route to display to a user. In order to come up with the best route, the software uses math algorithms and a decision model based on attributes like road closings, speed limits, number of traffic lights etc.
The software uses all this data and compiles to help it make a decision on the the best route. Therefore if you were to compare two competing gps systems side by side, you will probably get different routes depending on what routing engine the systems use.
There are two main players that design routing engine software for gps navigation systems, Navtexq and Tele Atlas. Garmin and Magellan systems use Navteq whil Tomtom uses Tele Atlas.
The software is sold to manufactures like garmin and magellan in packages and the price is based on how many attributes the software should analyze. Therefore more expesive gps systems typically have better routing engines than their least expensive couterparts.
One of the main reasons why your gps sometimes gives you funny directions is that its software did not process enough attibutes to give you the best route. This usually happens with the cheaper gps systems bacause the manufactures did not want to spend enough money on the routing engine. So the next time you get some funny directions from your gps system remember you get what you pay for.
Learn more about the best gps systems