@Mal
I would say yes to that, my understanding is each satnav has it's own routing algorithm, as does google maps, so when you create a route in MRA with few waypoints, each satnav uses it's own algorithm between waypoints so you can get differences in route driven.
MRA addresses this if you have a Gold subscription with the "Compare Route" functionality which highlights differences as you create your route and you just add extra waypoints until the routes are the same (as the differences show in the image above).
The basic subscription uses Openstreetmap, Garmins use the "Here" map, Tomtom uses it's own maps, MRA Gold gives you the choice of these maps.
What you need to do is increase the number of waypoints however as you are using the Openstreetmap, you won't be able to get your Garmin to follow the route exactly, you get as close as possible by using additional waypoints.
The route you created was pretty accurate to where you want to drive but chose the motorways because Avoid Highways wasn't an option in the freebie version of MRA
