Why is Risa going to be unwound?

Readers can find the answer to this question in the book's first section about Risa.  Risa is a "ward of the state," and she lives in an orphanage called a "StaHo."  She has lived in this kind of environment for her entire life.  She is fed and educated here, and the hope is that some family will want to adopt her.  That has not happened yet, and Risa is now fifteen years old.  Any StaHo is obligated to take in and care for wards of the state, but they are only obligated to do this through the child's thirteenth birthday.  After that, the child's fate is much less secure.  The headmaster of Risa's orphanage tells her that she is going to be unwound in order to make more room for the newly arriving kids.  He tells Risa that they have to take in the new kids and that there is not enough money to care for all of the older kids like her.  They have to cut 5% of the population.  

"The money only stretches so far," says the headmaster.
"Educational standards could be compromised," says the lawyer.
"We only want what's best for you, and all the other children here," says the social worker.
And back and forth it goes like a three-way Ping-Pong match. Risa says nothing, only listens.
"There are unwanted babies born every day—and not all of them get storked."
"We're obliged to take the ones that don't."
"We have to make room for every new ward."
"Which means cutting 5 percent of our teenage population."
"You do understand, don't you?"
Risa can't listen anymore, so she shuts them up by saying what they don't have the courage to say themselves. "I'm being unwound?"