The thread ID (in this case 10,000) doesn't accurately reflect the total number of posts on the board since new thread IDs are created when topics are split/merged/moved etc while the actual number of topics may go down (in the case of a merge) or stay the same (in the case of a move when a "redirect" is left behind in the old forum). In either case, the number on the index page that IPNixon pointed out is the "correct" number of topics that are actually on the forum.