Yes Eric that is where all the confusion started...! Royals virtually always take their titles from their father, through the male line. The Duke was made an Infante because, like you said his mother was. He also had a Italian dukedom (galliera). Here is the male line:
King Louis Philippe of France
I
Antoine, Duke of Montpensier
I
Antonio de Orleans-Galliera y Borbón
I
Infante Alfonso, Duke of Galliera
I
Alvaro, Alonso and Ataulfo
You can now see why everyone is going on about a french title.