Limitations with GDP and how it is calculated is by many factors that show a positive transactions and negative. Black market transactions, Bartering , and volunteering are not included when calculating GDP. Crime, economic disasters, and divorce are treated as an economic gain. A disaster like an oil spill is caculated as a GDP gain. These factors mess the actual GDP caculations of the nation and leads to different numbers.
-From Jay Hanson, http://dieoff.org/page11.htm