Most of the new Garmin units will do a recalculation when a route is moved from Basecamp to the device, and recalculation can't be turned off, at least on the standard units.
The only solution I've found is to use lots of waypoints. The problem with that is unless you are careful ( and magnify enough to be exact), the added waypoints may not be exactly on the same road as the route, addind lots of "jogs" to the route. As others have stated, be sure you have the same "avoides" set up on each.
All in all, I find Basecamp fustrating enough that I rarely use it. I try to lay out a route on secondary roads & it constantly tries to get me on parallel roads that are "faster".