If it's any consolation, I get similar results with my garmin nuvi 205w and I have got used to that; it's still an annoyance though when you can see on the map that there is a route that you think is a better choice but the Garmin knows better.
It does as you describe with either the OSM selected or the inbuilt garmin map or when using a purchased garmin map (on a SD card) that contains the whole of Europe, so I have assumed that it is to do with the garmin operating system.
I have tried fiddling with the garmin settings - avoid this, that and the other - but it appears to make no difference to how the garmin selects a route.
I had been considering making a post like yours, but I have just got used to how the damn thing works and I kind of ignore it except for getting around cities/towns that I don't know. I'll be interested to see if anyone has better ideas about how to use a garmin.