TomTom City Maps

Anyone got this? I've just got it and noticed something a little odd. I loaded the Liverpool Area map and tried it out calculating a route. It was pretty quick and check 16,000 roads in order to create my route. I then loaded The Greater London map and had a play about. I switched back to the Liverpool map and re-calculated the same route, but this time it took a lot longer and checked over 50,000 roads. Seems a bit strange that it checks roads in maps that aren't "loaded" and aren't even connected. I was going to load a few more maps but not 2 sure if that's such a good idea now. anyone else notice this?