I think it has to do with the new “dust payout” elimination feature. It raises the difficulty enough to minimize transaction fees when spending mined coins. It appears you may be able to reduce the share difficulty again by adding “+n” to the miner username, where n is the desired share difficulty. However, you may incur heavy transaction fees in the future when you go to spend the mined coins.

Maybe someone can confirm this for sure. I've been finding it difficult to find detailed p2pool documentation lately for some reason.