The media reports...and people believe without fact checking. 

"Mexico recovers a lot more guns than it submits to the U.S-nearly 29,000 recovered by Mexican authorities" (and) ..."Mexico submitted 7,743 firearms for tracing..." same time frame.

So *Specifically* and factually,  "90 percent of the guns *submitted for tracing* by Mexican authorities were then traced to the U.S. ..." 

 "The percentage of all recovered guns that came from the U.S. is unknown... 

The president isn’t the first to make this mistaken claim; far from it... The 90 percent figure was similarly cited by Sen. Dick Durbin (D-Ill.) and Sen. Dianne Feinstein (D-Calif.) ...And it’s been reported by a phalanx of news organizations, including the Christian Science Monitor, the New York Times, NBC and the Chicago Tribune ... 
www.factcheck.org/2009/04/counting-mexicos-guns/

PS Many illegal Mexican guns come from their southern border neighbors...btw.