Also, many Central Americans (and some South Americans) come into Mexico en route to El Norte: most of them don't make it. Freight trains into Mexico from Central America have often carried more stowaways on their cars' roofs than passenger trains carry inside.
Perhaps some of that Mexico-U.S. immigration stream includes the Central Americans who do make it, but I don't see any representation of the influx into Mexico.
AbE:
Edited by Omnivorous, : No reason given.
"If you can keep your head while those around you are losing theirs, you can collect a lot of heads."