Thank you all for your help! I’ve finally figured it out and now understand how overlays work too. It seems I’ve been using the 'Here' map all along, which is ideal for Garmin. However, the two maps below highlight the issue I was asking about.
The first picture shows the route I created on my PC using MRA and the 'Here' map for Garmin.
The second picture shows my Garmin map and the deviation it has taken - this I guess was done on transfer?
I actually ignored the deviation and continued on the original route, there was no reason for the Garmin to take me down the route shown?