An endless pool of prisoners

"Why is Israel releasing 440 Palestinian prisoners specifically ahead of the Annapolis conference, and not 500 or 300, or 2,000 as the United States had expected? The impression is that no one is exercised by the security risk entailed in releasing prisoners - aside from politicians who want to make political capital off of it - and that all the wheeling and dealing revolves around the question of how many prisoners "are worth wasting" on this or that event."