In regards to The Absolutely True Diary of a Part-Time Indian, why does Alexie call his book the diary of a “part-time” Indian?

Alexie titles his novel The Absolutely True
Diary of a Part-time Indian because the phrase "part-time" refers to the
themes of isolation and cultural conflict around which Junior's story revolves.  Junior
sees many members of his tribe on the reservation become complacent with the low
standard of living that they have, and when he decides that he wants a better education
for himself, he finds that the Indians around him offer little support.  They call him a
traitor for wanting to go to a "white" school.  But even after Junior leaves the school
at Wellpinit to attend Reardon, he cannot leave behind his Indian culture and identity.
. .nor does he want to.  Junior sees beauty in the reservation and wishes that the
socioeconomic reality of the reservation were equally as beautiful.  At one point in the
novel, Junior draws a picture of himself divided in half, and this self-representation
characterizes what Junior sees as being a "part-time" Indian.