Yeah LOL that's what confused me, the change at 21,168. So in effect, when the fork kicked in at 21,000, instead of changing at 22,176, as was originally coded in the original release, it basically took the last difficulty change as being 20,664 (even though it didn't change at the time), and since the next difficulty change would have been 21,168 (20,664 + 504), that's why it changed. I didn't think about that at the time...