If you join an event late then you can get the route achievement (in this case Innsbrukring) if the event completes a route even if you have only done part of the route.
Nice hack! 
It probably isn’t linked to events, I rode Duchy Estate for it’s achivement this morning, and upon leaving the course after getting it went the wrong way, the long climb instead of the reverse KoM. I U-turned just after the banner and got the harrogate circuit as well.