User blog comment:YazzyDream/Oda Going on Break After Chapter 597/@comment-1604252-20100825162851/@comment-1595565-20100825210351

There probably a filler of some kind. The anime tries to stay behind the manga by a certain margin, and uses fillers if and when they start to close in too much. The last thing the anime people want is for the anime to catch up or even come close to catching up with the manga.