It may be because of our "one to many" relationship for ZIP codes. For example: 08512 Cranbury, NJ and 08512 East Windsor, NJ both route to the same spot on the map. 

If you actually want to go to East Windsor you have to route to East Windsor, NJ with no ZIP. East Windsor also shares a ZIP with Hightstown which makes it doubly hard to route to 08512 East Windsor, NJ and 08520 Hightstown, NJ.