From the last paragraph of the excerpted article posted above:
"In addition to the flurry of new claims, an additional 13,000 Mexicans entitled to benefits but cut off by provisions in recent immigration laws could also begin receiving their checks. In a 1996 immigration reform law, Congress decreed that foreigners not legally residing in the United States could no longer claim benefits, unless their home countries were subject to a treaty.
" Gabriela Lemus, director of policy and legislation for the League of United Latin American Citizens, said as much as $21 billion in Social Security payments have not been tracked to potential beneficiaries, most likely because they were paid under a false Social Security number. "