My Garmin is a total exercise in futility a large amount of the time.
Same problem as you describe. It will guide me down a narrow residential road littered with stop signs and small traffic islands that runs 1 block parallel to a main road. It makes no sense at all.
It has almost become a game to plot all routes on my Garmin even when I know the area like the back of my hand, just to see what the thing does.
So many times I will plot a route and it have me go past my destination, tell me to make a U turn and come back to where I just was rather than just turning left into the parking lot of where I am going. Absolutely asinine!!!
I think the route planning department at Garmin spends to much time drinking their own bath water......
I should have stayed with my Magellen. The route planning on it was far superior to Garmin.