The post payouts are estimated using a 3.5 day average of the Steem price. The reason active post values can drop is due to the fluctuating price of a Steem and SBD.

So, posts dropping a bit is normal.

I don't know if you mentioned it, but you should save your vote bots for use in the last day or so before payout if you want to maximize how close the estimated payout will be.