Almost every GPS based navigation device does its own routing on the fly. The device has its own map information, which includes routing priorities, and uses at most only the stops identified on the route imported.
I found this to be the case even when using identical map data on the computer, i.e. map data in the GPS device came from the computer, and the routing program came from the GPS manufacture.
In the case of Good Sams online planner and Rand McNally's device, it is unlikely that they have map data in common to the extent they might share routing details. The work around is to put in enough stops or waypoints to force the GPS to come up with your preferred route.