Mamerto Arcala - Discrepancies

Mamerto "Jimmy" Agan Arcala

After further research on my grandfather, Mamerto Agan Arcala, son of Bartolome Arcala and Narcisa Arribas, I have found a 1940 Census record that lists him as Mamerto Arribas and that Pete Arcala was his step-brother.  This does prove that there are discrepancies in Mamerto's family tree.  First of all, his death records and his naturalization record documents his legal name as Mamerto Agan Arcala.  By tradition, Filipinos take on their mother's maiden name for their middle name.

Who was Agan?  Why did Mamerto use the surname Arribas instead of Arcala in the 1940 Census?  Was he not biologically an Arcala, but took on the Arcala surname to match his brothers?

If Mamerto was an Arcala, was Bartolome married to another person with the surname Agan, other than Narcisa?  Or was Mamerto the son of another Arribas, relative of Narcisa?

Personally, I have now seen and/or heard my grandfather's name in multiple ways:

1. Mamerto Agan Arcala (Who is Agan?  Was that his biological mother's surname?)
2. Mamerto Santiago Arcala (Santiago was his deceased brother in Washington, USA), hence how Mamerto got the nickname "Jimmy" for Santiago translates to Saint James, and Jimmy is the nickname for James.
3. Mamerto Arribas (Was he named for Narcisa Arribas whom was the mother of Antonio Arribas Arcala and Pedro Arribas Arcala, or another relative by the same surname?) Mamerto did state his mother's name was Narcisa Arribas on his marriage application to my grandmother Lenore Evelyn Barron.  However, they also both lied on the marriage application about their ages.  

His name is documented as Mamerto Agan Arcala on his Naturalization Record.

He lists Narcisa Arribas as his mother on his marriage certificate, which he lied about his age on.  My grandmother Lenore also lied about her age.

Mamerto's surname is Arribas on this 1940 Census and lists his brother Pete as his "step-brother."

• Hilarion Arcala (aka Pedro) was married to Macaria Agosto Quibal on July 7, 1917 and his parents were listed as Bartolome Arcala and Narcisa Arribas.
• Antonio Arcala was married to Gertrude Daudin (neé Lueckernan) and lists his parents as Bartolome Arcala and Narcisa Arribas.  The Social Security Death Index states his mother's maiden name was Agan.
• Santiago Arcala dies in Tacoma, Washington on July 2, 1925 and his parents were listed as Bartolome Arcala and Narcisa Arribas.