How does the confusion over Junior's name demonstrate his change in culture in The Absolutely True Diary of a Part-Time Indian by Sherman Alexie?

In the chapter titled "How to Fight Monsters," Junior arrives for his first day of classes at the school off the reservation, Reardan. When he tells Penelope his name, Junior, she and her friends laugh. Junior reflects on the situation:

I had no idea that Junior was a weird name. It's a common name on my rez, on any rez.

You walk into any trading post any rez in the United States and shout, "Hey, Junior!" and seventeen guys will turn around.

And three women.

But there were no other people named Junior in Reardan, so I was being laughed at because I was the only one who had that silly name.

And then I felt smaller because the teacher was taking roll and he called out my name name.

"Arnold Spirit," the teacher said.

He moved on to other students, but Penelope leaned over toward me again, but she wasn't laughing at all. She was mad now.

"I thought you said your name was Junior," Penelope said.

She accused me of telling her my real name. Well, okay, it wasn't completely my real name. My full name is Arnold Spirit Jr. But nobody calls me that. Everybody calls me Junior.

Well, every other Indian calls me Junior.

"My name is Junior," I said. "And my name is Arnold. It's Junior and Arnold. I'm both."

This scene shows one of the many cultural shifts Junior will encounter as he moves from the rez to the white school off the reservation.  At home, he doesn't have to explain that he has two names; in fact, a nickname is something common throughout the country.  But the very basic name of "Junior" is something specific to the reservation, and Penelope feels affronted because she thinks that Junior has somehow lied to her about his name.  

Junior will continue to encounter more cultural confusion in his year at Reardan, including an incident with a group of jocks who have been picking on him since he started.  Now, Junior is used to be picked on; he met his best friend Rowdy because he was being bullied on the rez.  So, Junior's rules of fighting turn out to be very different from the unspoken rules at Reardan.  When he takes the first swing and knocks out the biggest jock of them all, Roger, he surprises all of them, including himself.  He actually earns respect from the jocks, whereas on the rez, he would have been an even bigger target to be bullied and beaten up.