Good point!
From memory (again - its not my unit) the route looks ok for everyone when compared in Basecamp but when moved over to the 590 unit the mileage differs, or the mileage seems to change once the route is actually started. One of the main problems seems to be the 590 wants the user to do a u turn as if its missed a way point. Its normally at this point the 590 gets switched off and we rely on my 660 only. This has happened on a number of occasions. As i say - the maps and routing preferences should all be the same.