I'm sure the agency will tell you there are a number of reasons they sometimes can't remove people. They either can't get the documents to remove them, or it's not safe to remove them to their country, or various other reasons. That would be up to the agency to explain. But they do know the whereabouts of 22,000, and there are a further 41,000 they do not know the whereabouts of.
On May 8th, 2008. See this statement in context.