Usually it's a case of Shipped vs. Sold.
If Sony says they have sold 33.5 million, they have sold these to shops all over the world. Not to actual consumers, which is what VGChartz is tracking.
The downward adjustment of the PS3 was due to it being overtracked in the Others region. Apparently Ioi had made certain assumptions that were not in line with what the more established tracking firms was assuming. Or something like that.
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