I think the avoidance is the issue.
You can define the curvature per section, but the avoidance is a global setting for the route.
Kurviger tries to find the fastest route avoiding highways. The avoidance is set to 5 = maximum. So highways are avoided, if there are alternatives.
You can either play with the strength of the avoidance or better switch off avoidance and trust Kurviger 
Some additional shaping points - if necessary - should do the job.