No consumer road mapping GPS device I am aware of is 100% accurate as far as route planning - they all make mistakes. That is why we need to carefully study the planned route to make sure it is taking us the way we want to go. This is not as difficult as it may appear at first.
In some cases the device will have us get off a highway to go around in circles and then return to the highway at a different point.
We recently crossed from Canada to upstate NY. The GPS had us go into NY and then back to Canada and then back to NY. It also wanted us to take a ferry over Lake Champlain.
We have long used Microsoft Streets & Trips for planning routes as it is much easier to see the route and to make any changes. We can put in "stops" or "waypoints" to create the route we want to take. We have planned one day drives in Mexico where we had to put in up to 13 waypoints. That would be from Lake Chapala to San Miguel de Allende without taking Cuotas.
With some Garmins and some versions of Streets & Trips you can plan a route in S&T and then connect your Garmin to your computer to have the Garmin take the route you planned in S&T. I have not done this but others have. Another way is to plan the route in S&T to figure out the stops to force the route and then manually add those stops to your Garmin.
We can also plan and force a route in the Garmin but it is not as easy. We have to study the map on the Garmin and move it around to track it all down and then put a point on the map for each stop or waypoint - we touch the map at each point in the correct order and have each one added to the route. Once a route is created it can be named and saved for future use.
There are lots of things that can be done with a GPS and lots of mistakes that can be made. Fooling around with the device until we have a firm grasp of ways to use it is time well spent. It can keep us out of trouble - and get us out of trouble we might get ourselves into.
Never ever follow a GPS blindly. You must check the route and plan the route. There are ways of doing this but you have to learn before you hit the road.
https://en.wikipedia.org/wiki/Waypoint
A waypoint is a reference point in physical space used for purposes of navigation, otherwise known as a landmark.