edjevink on 04 February 2010
The reported 33.5 million units is the total amount of PS3's that Sony shipped to retailers. The 30.85 million units that VGChartz has is the total amount PS3's that those retailers sold to real customers like you and me.
BUT, the difference in shipped units and sold units is way bigger for PS3 than for X360 and Wii, so it could be that PS3 is indeed undertracked on VGChartz, or that X360 or Wii are overtracked.
