It's a matter of what data was used to make the maps, the main difference between nav systems lies in the data source used for the map info - it can vary hugely from one system to another.

If you don't have the latest map disc, you might consider upgrading to the newest version, as mapoping companies continuously refine their map data and correct errors and add more streets.

You are luckier than I as you can get newer versions. In my country, the most recent version published by BMW is January 2005, now 2.25 years old....