The two-surname system creates discrepancies because different documents handle names differently. Here are the main reasons a person's name may appear in several forms across their document packet.
U.S. forms have one "Last Name" field: When a Mexican-born person fills out a U.S. immigration form, they encounter a single "Family Name (Last Name)" field. Some people enter both surnames ("Ramírez Torres"), some enter only the paterno ("Ramírez"), and some enter them hyphenated ("Ramírez-Torres"). There is no single correct way to fit a two-surname name into a one-surname field, which means the form will almost certainly show a different name than the birth certificate.
Passport name formatting: Mexican passports often display the name as "RAMIREZ TORRES DIEGO ALEJANDRO" (paterno, materno, given names — all in uppercase, no accents). This is a different format from both the birth certificate and the U.S. immigration form. The passport may drop accents (Ramírez becomes RAMIREZ), which is technically a different character but the same name.
Married name conventions: In some Spanish-speaking countries, women may add "de [husband's paterno]" after their own names when they marry. So "María Elena García López" might become "María Elena García López de Hernández" on her marriage certificate or voter ID. In the United States, this same person might use "Maria Garcia" or "Maria Hernandez" or "Maria Garcia-Hernandez" on different documents. This is the same person — but the names look dramatically different.
Accent marks and special characters: Spanish names frequently use accents (á, é, í, ó, ú), tildes (ñ), and diereses (ü). U.S. government systems often cannot process these characters, so they get dropped. "Ramírez" becomes "Ramirez," "Nuñez" becomes "Nunez," "Álvarez" becomes "Alvarez." These are not errors in any individual document — each system is doing its best with the characters it supports.
Generational changes in naming practice: Older documents from some regions may list only the paterno, or may use different ordering. A grandparent's birth certificate from 1940 might follow slightly different local conventions than a grandchild's formato único certificate from 2020. Both are correct for their time and place.