Stockpiling is a standard tactic, especially for conservative business plans. The reasoning is simple: if a product is unavailable when demand is highest, more sales will be lost than if a product is unavailable when demand is lowest. Working off of the various figures Nintendo has given, it seems like they try to keep at least 2 million units stockpiled at a time, and up that stockpile whenever possible in the time leading up to major releases and holiday rushes.
I did a rough calculation based upon what dates I remember Nintendo stated they upped shipments, and came up with this possible distribution:
Launch: 1.0m units available
November 2006: +1.0m (2.0m; sell-through: 0.5m; 1.5m unsold)
December 2006: +1.2m (3.2m; sell-through: 2.9m; 0.3m unsold)
January 2007: +1.2m (4.4m; sell-through: 4.0m; 0.4m unsold)
February 2007: +1.2m (5.6m; sell-through: 4.8m; 0.8m unsold)
March 2007: +1.2m (6.8m; sell-through: 5.8m; 1.0m unsold)
April 2007: +1.2m (8.0m; sell-through: 6.7m; 1.3m unsold)
May 2007: +1.5m (9.5m; sell-through: 7.6m; 1.9m unsold)
June 2007: +1.5m (11.0m; sell-through: 8.9m; 2.1m unsold)
July 2007: +1.5m (12.5m; sell-through: 10.0m; 2.5m unsold)
August 2007: +1.8m (14.3m; sell-through: 11.4m; 2.9m unsold)
September 2007: +1.8m (16.1m; sell-through: 12.6m; 3.5m unsold)
October 2007: +1.8m (17.9m; sell-through: 13.3m; 4.6m unsold)
November 2007: +1.8m (19.7m; sell-through: 15.7m; 4.0m unsold)
December 2007: +1.8m (21.5m; sell-through: 19.3m; 2.2m unsold)
January 2008: +1.8m (23.3m; sell-through: 20.6m; 2.7m unsold)
February 2008: +1.8m (25.1m; sell-through: 22.2m; 2.9m unsold)
March 2008: +1.8m (26.9m; sell-through: 23.5m; 3.4m unsold)
April 2008: +1.8m (28.7m; sell-through: 25.0m; 3.7m unsold)
Even if it's not 100% accurate, you can still see the stockpile effect taking place. Each month leading up to the holidays, there's a gradual increase in units unsold, which drops dramatically by the end of December, then begins to slowly rise again.
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