Why doesn't my dad call my sister by her nickname?

My parents and my grandmother have called me Maddie since I was born and it's stuck ever since (although some of my teachers at school call me Madison). However, when my sister was born, my mom and my grandmother gave her the nickname "Isa" and my dad still called her Isabella, despite everyone around him using the nickname. When he refers to the two of us, he says "Maddie and Isabella." Do you have any idea why he might do that? Unlike me, my sister despises when someone uses her full name instead of Isa, so this has come to annoy her.