New evidence suggests that shifting production overseas has inflicted worse damage on the U.S. economy than the numbers show. BusinessWeek has learned of a gaping flaw in the way statistics treat offshoring, with serious economic and political implications. Top government statisticians now acknowledge that the problem exists, and say it could prove to be significant. The short explanation is that the growth of domestic manufacturing has been substantially overstated in recent years. That means productivity gains and overall economic growth has been overstated as well.