I don't know what chart you're looking at, but the first 13 months of the PS2 SMOKED the PS3 in North America.
http://vgchartz.com/hwlaunch.php?cons1=PS2®1=America&cons2=PS3®2=America&cons3=PS3®3=America&weeks=56
If fact, if you compare each region separately from launch, the PS2 completely obliterates PS3 sales.







