What we usual see when we open the poser scene is the North side , b/c it is usual to make a good pictures when the sun light light up the scene right?  if you take a picture with the sun behind you it would come out not that best.. The HDR maps caching the light as it show up in the real world, if you make a nice lighted up picture for your poser scene it would be the north side.. but the reflection in the mirror ball will show the south side .. here one more time the poser scene as it show up on start, the ball is mapped with a sphere map ( Latitude/ Longitude ) 2:1

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"Surrender to what it is - Let go of what was - Have faith in what will be "