This is because steem value is a 30 day average. That's why when they switched to the 30 day average and did not pre-calculater the pool that the payouts and prices really sucked for about a month. So a sudden change in price of steem will not have an instaneous dramatic impact on the payouts... it will pull that 30 day average down. If the down tick lasts for a bit then the average will trend down as well.