If you attempt to launch the game and it proceeds to download an update you already have or the full installer when it shouldn't be abort that download and proceed to launch the game offline (double the World War ][ Online application). This should get your current installed version registered correctly with the OS and allow the correct update process to take place.

You will likely have to do this after every update install until such time as the issue is corrected in the PlayNet downloader or Mac OS X (this issue showed up after a security update for Mac OS X that changed how Launcher Services deals with applications that users haven't yet run).