We ask this question over and over again. The ambassador and I were at the same meetings. He may have additional information, but in general we could not get a firm answer as to what the criteria are that cause some to be kept in and others to be released. The impression I got, and it's only an impression, is that a significant majority are released at the end of the 18 months, but that is only an impression and it could be wrong.
I also get the impression that people who belong to the more nomadic, pastoralist populations for whom it would be a meaningful hardship for them to be away from their families are normally released quickly so they can return to their communities. I was told specifically that if a woman develops a relationship and wants to get married, she can be released. The negative side of that may be the basis for Mrs. Grewal's concern about the treatment of women.
However, these are all, unfortunately, our speculations based upon the speculations of those whom we asked. People seemed sincere in their efforts to give us concrete answers, but they were unable to do so.