It took me a while to figure out the problem, but it is clear that Garmin filters out parking areas from the map in the Garmin Explore app.
This is very annoying because certainly here in Belgium lots of those places are used for walking/cycling. The result is that you get lots of extra distance and the use of major roads although there is a much better alternative available!
When I know the area I can switch to a direct route for those pieces, but when the area is unknown you just can't guess..