@Crispey happy to see - you have detected the difference in the routing engine (list item in "Options") and the map overlays (list item on map).
The map overlays don't have any affect to route planning. It's more gore your comfort to read the map.
The list items in "Options" makes a difference in routing and gives you the possibility to compare, how your route will looks like if you select:
Standard (Open Street map)
TomTom
HERE (MyRouteApp, Garmin)
If you would like to share your route with others (my they uses TomTom) - and you can see, your route changes if you switch from Standard to TomTom - then you can enter more routepoints to force the engine to create same route by both options.
If you want to use MyRouteApp Navigation for your ride, you need to plan it with engine HERE, because this the only, the App is navigating a route!