witadmin importgloballist fails but says it succeeded

I'm running witadmin from the Visual Studio 2013, Update 4.

I'm importing a new global list.

I run witadmin as follows;

c:\program files x86)\microsoft visual studio 12.0\common7\ide\witadmin.exe importgloballist /collection:http://<servername>:8080/tfs/<collection name> /f:<path to global list.xml>

The program says:

The global list was successfully imported.

I then run 

c:\program files x86)\microsoft visual studio 12.0\common7\ide\witadmin.exe exportgloballist /collection:http://<servername>:8080/tfs/<collection name> /f:<path to global list.xml>

The changes have not been made.

After much trial and error, I finally determined to delete the TFS cache by typing:

%localappdata%\microsoft\team foundation server\5.0\cache

I then run the witadmin to import the global list and it works!

Why do I have to delete the local cache to get it to work?  Why can't I rely on the result message from witadmin to tell me the truth?

- Bruce

Bruce