From this article it seems that some defective systems are returned to the retailer and in this case they would get double counted in the sales numbers. Also it shows that even 6 months after the launch of Falcon the number of returns was still high since those were probably non-falcon systems. If 30% of the systems sold before falcon (sep 07) were returned to the retailer, then 16.4% of 13M boxes or 2.2M were returned and therefore double counted. I am not saying anything about number after sep 07.
In February 2008 an examination of 1040 Xbox 360s by SquareTrade found a 16.4% failure rate; 171 were returned under warranty as "disabled", 60% of which with general hardware failure.[19][20]. However SquareTrade also admits that their estimates are likely much lower than reality, due to many owners of failed consoles who are getting them repaired directly via Microsoft, they also note that the consoles were only tracked for 6-10 months, and in the longer term, many more consoles will have failed. This ties in with most other sources claiming 30%-40% failure rates. [21]
