That number had no validity either. The CORRECT statement was that 90$ of guns the Mexican government ASKED the ATF to trace originated in the US. They didn't bother asking unless they had reason to believe they originated in the US. In short, the ACTUAL meaning of the statistic is that 10% of the guns the Mexican government THOUGHT come from the US did not.
Note: The 40% number came from a SURVEY of gun owners, not from records of sales.