Why is Junior surprised when Penelope gets upset about him telling her his name?

In the book "The Absolutely True Diary of a Part-Time Indian" by Sherman Alexie, a white girl named Penelope asks Junior his name. When he answers "Junior," Penelope and his girlfriends laugh. Junior finds this sort of behavior strange and explains to them that there are 17 people by the name of Junior on the rez.

Mr. Grant starts the roll call and junior answers to the name of "Arnold Spirit." This confuses Penelope and she gets upset that Junior did not tell her his real name. Junior is surprised as he has never received this sort of attention from a white girl before and this makes him feel special in a way. He says the lines ""My name is Junior," I said. "And my name is Arnold. I'm Junior and Arnold. I'm both." I felt like two different people inside of one body. No, I felt like a magician slicing myself in half, with Junior living on the north side of the Spokane River and Arnold living on the south." to convey to her that both Junior and Arnold are his names.