I created a route on basecamp between two locations I transferred to Garmin Nuvi 2597 I check on the device and the route has transferred OK. When the trip has started and for what ever reason (maybe I have stopped for fuel etc) the route is recalculated and results in a different route between the two locations
If you go off route then your nuvi will recalculate as that's how it works. Otherwise provided you don't force a recalculation it will follow the route sent by BaseCamp.
If it's important that certain sections of the route must be followed then insert sufficient viapoints to ensure the recalculation options are limited.
Just checked options in basecamp Routing has avoid Residential Roads but Nuvii does not Unchecked this selection and re transferred route and now both basecamp and Nuvvi have the same routing