Nintendo lowered their Wii shipment forecasts by 1 million

[NJ5]

Mr Khan said:

Personally, i think the shipment forecast is simply about their inability to get production up to quite where they expected it to. They also cut their financial forecast, which is more due to the strong yen against the weak dollar and pound sterling.

That could also explain why December NPD sales were lower than November's (per week), so it's a sound theory. As I recall it, they were hoping to increase production slightly for the holiday period, so maybe they didn't stockpile enough units.