Sales of new homes dropped unexpectedly in September to a seasonally adjusted annual rate of 402,000 units but the inventory number sank to the lowest level in 27 years.

According to a new research note put out by IHS Global Insight (IHS), the decline actually might have been caused by builders not having enough completed homes on hand to sell.

IHS noted that the benefit caused by the $8,000 first time homebuyer tax credit has passed. Figures released by the Commerce Department show that sales fell 3.6% compared to a downwardly revised rate of 417,000 in August.

IHS reported that inventories "sank for the 29th straight month" but added that "single-family housing starts, however, will probably continue to grow because builders, at some point, will need to ramp up production, or they will lose sales."

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