The 5.5 million cites number "sold" by Sony from their Annual report (fiscal year 2006), just like the 5.84 million is the number used by Nintendo.
From your second source, ssj12:
"In its annual report, Sony said it sold 5.5 million Playstation 3 units in the fiscal year ended March 31 2007, despite having faced production yield problems for its blue semiconductor lasers that feature inside each machine."
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The Movie Thread: http://www.vgchartz.com/forum/thread.php?id=6880
The Crow Eating Thread: http://www.vgchartz.com/forum/thread.php?start=0&id=3886
The Betting Thread: http://www.vgchartz.com/forum/thread.php?start=0&id=7104
Custom GIFs Thread: http://www.vgchartz.com/forum/thread.php?id=18963
The Greatest Game Ever Conceived On Any Platform
Tag: "I have tasted Obi-Wan's bitter tears"