In addition to the materials you must also pay attention to the "Function" of the layer. The selections on this first column in the wall type editor do make a difference as they assign additional priorities to the clean-up solution.
In the attached image:
Wall type 1 has the same wall materials for the interior and exterior layers and are assigned to the same function: Finish 1 [4], with an air layer assigned to the Thermal/Air Layer [3]
Wall type 2 is the same except that the exterior layer is assigned to the function: Finish 2 [5].

By assigning the exterior wythe to a different priority we tell Revit to treat that layer differently. Play with those values (Function and material) and you will see the many different combinations that are achievable.
Gary J. Orr
(Your Friendly Neighborhood) CADD/BIM/VDC Applications Manager
aka (current and past user names)
GaryOrrMBI (MBI Companies 2014-Present), Gary_J_Orr (GOMO Stuff 2008-Present); OrrG (Forum Studio 2005-2008); Gary J. Orr (LHB Inc 2002-2005); Orr, Gary J. (Gossen Livingston 1997-2002)