When performing a route analysis using the hierarchy option, the route returned is not optimal -- there is another route that is much better than the route that was returned.
When using hierarchy, the route is determined with the solver favoring the primary roads (hierarchy level 1) over the secondary roads (hierarchy level 2), and the secondary roads over local roads (hierarchy level 3). Also, the solver tries to travel as much as possible on hierarchy level 1.
This is a heuristic approach, which is sensitive to data quality and modeling. For instance, if the hierarchy level 1 is sparse or is disconnected, it may result a long route. In the worst-case situation, the solution cannot be found.
Note: Barriers placed on the network have a special treatment and do not cause disconnects in the hierarchy levels at solve time.